I'm surprised that there were so many unlisted control letters as Mexican notes have a pretty healthy following. Maybe less so with these specialized issues. It's interesting that this lot contained such a diverse selection of control letters, was this a former collection or a random lot pulled together by the seller?

Tracking serial numbers can be very useful/insightful, I'm sure you're doing the same but I keep a spreadsheet of certain notes listing known serial numbers, grades, sell prices, etc. and catalog images. I'll "dumpster dive" all auction sites and their archives, takes a while but it's time well spent. I would think you could have a pretty good list of these notes in short order. 

How many different sets of control letters do you suspect were used and was the vertical and horizontal orientation sporadic or a permanent change to how they were applied?

My thoughts exactly and there does seem to be plenty of information available for certain series e.g. the dos caritos issues, however I was also surprised at the many gaps in the data for a popular collecting area like Mexico - having said that the data on the fractional real coinage has only recently been improved with the excellent books by Brad Yonaka. There are even plenty of discoveries to be made in Napoleonic issues where you would think the records were detailed and available.

There seems to be even less info for the rest of Latin America, which are the only ones I have looked at so far, but I expect it is the same for most of the world - Ukraine??. Although I expect individual collectors may have the data somewhere - like you I tend to (well try to ) record such information and this is now much easier with Ebay and auction records available online. I should say that Simon Prendergast, at the US Mexican Numismatic Association (usmex.org), has been great allowing such findings to update the central records.

Back to the Durango 5 peso issue M1495, there are no graded examples (which probably doesn't help) with the best that I am aware of being two notes in VF with the rest falling into the 'tatty' category. I think the code letters were changed every 2000 notes with the overall sequence repeating every 12000 notes, for example:-

UGT/ITE (vert)*
OIY (vert)/L-AR (vert)
LPS/EID
ERO-/MIR/GON
ERA-/GFA
MA-A

*Almost all relevant blocks that I am aware of largely have a single set of control letters, in a consistent position on the note, however it appears that some numbers (last ~20000?) were issued at different times and it gets a bit random at that point e.g.  should the single example of ITE(vert) actually be in the next block which would then have all the vertical control letters together. I need to find more examples to confirm/fill in the gaps. As you suggest I think this group of notes was a collector trying to put different examples together as this range in a natural accumulation would be extremely unlikely. Several other group lots that I acquired were also made up of a range of series/control letters etc - I wish I had also got the last lot that was available!

[some other Mexican issues change every 2000 and both horizontal and vertical control letters are known]

On 3/28/2022 at 3:44 PM, ColonialCoinsUK said:

There seems to be even less info for the rest of Latin America, which are the only ones I have looked at so far, but I expect it is the same for most of the world - Ukraine??

Surprisingly, Ukrainian banknotes are pretty well documented, at least much more so than I thought when I started collecting them. I now have over a dozen individual references ranging from 1953 to 2021, the only problem with most of these is that I have to translate them from Ukrainian, Russian, German, ect. to English. What a pain. While none of the references contain all of the information, most of it is there to to be found. That's not to say there are not things to be discovered or misinformation corrected, but blocks of missing control letters for a single series as you have found would be unthinkable to me with Ukrainian notes, as that should be basic/commonly known information.  

For the control letters, you have 12 listed above, I find it less likely that multiple combinations were used within groups of 2000 and the sequence restarted after 12000. (My interpretation of your info above) I'm assuming that you have notes with the same control letters spanning a number range that includes notes with different control letters in it.  This is a very interesting project that you've taken on, I like to think that there will be some type of pattern to the control letters that you will unlock but more often than not the answer turns out to be... just because. 

Languages have never been my strong point but I can figure out some Spanish names and numbers - I couldn't even attempt anything with a different alphabet so Asian languages etc are completely out, Russian, German and Ukranian is impressive . My excuse is that my brother must have got all the required genes as he speaks multiple languages and even does some translation work on the side.

So with my limited vocabulary it looks like I have 18 notes covering 15 blocks and there are only an extra 7 blocks that I am currently aware of - yes a whole SEVEN notes. Given an issue of 100,000, albeit with alot of circulation, I would hope that a few more examples survived and they are out there somewhere. The other denominations and series appear to have even less examples!!

A sequence would be:-

00001-20000 ?
12001-14000 UGT
24001-26000 ?
36001-38000 UGT
48001-50000 UGT
60001-62000 ?
72001-74000 ?
84001-86000 UGT
96001-98000 ITE (vert)?

I would guess/predict notes with numbers in the missing blocks also have UGT as the control letters. For comparison 58-60K, 70-72K, 82-84K and 94-96K are all MA-A so the earlier blocks 10-12K, 22-24K, 34-36K and 46-48K may also be MA-A or at least all be the same. Could the apparent blocks of 2000 notes correspond to how they were shipped from the printer? there are examples where this happened rather than just consecutive numbers making up a shipment - another security measure? or it could all be rubbish.

How to crack the code? In the dos caritos 1 peso series some control letters are VILL-A, SCO-N, NCI-M, and FRA-L. So this would be FRA-NCI-SCO-VILL-A otherwise known as Pancho Villa - the only person I was familiar with before looking into the history of the period. In the 5 pesos dos caritos series some of the control letters seem to be the initials of the leading insurgents at the time F-V (the infamous Francisco Villa again), J-E-R (José E. Rodríguez) E-A-B (Eugenio Aguirre Benavides) etc, etc.

So names, initials, places etc seem to be an option - is L-AR referring to Leon (Domingo) Arrieta the leading revolutionary and short-lived Governer of Durango and is MA-A his brother Mariano Arrieta??

I am so glad that things are now electronic with 'saved searches' so it is now a waiting game and maybe a few more gaps will be filled in over the next 30 years or so.

On 3/29/2022 at 4:26 PM, ColonialCoinsUK said:

Languages have never been my strong point but I can figure out some Spanish names and numbers - I couldn't even attempt anything with a different alphabet so Asian languages etc are completely out, Russian, German and Ukranian is impressive . My excuse is that my brother must have got all the required genes as he speaks multiple languages and even does some translation work on the side.

Google is the source of my translations and then me translating whatever the google interpretation was. I'm fluent in google though.  My reference books are a fright, the translations/notations within and around the text is something to behold.  

On 3/29/2022 at 4:26 PM, ColonialCoinsUK said:

So with my limited vocabulary it looks like I have 18 notes covering 15 blocks and there are only an extra 7 blocks that I am currently aware of - yes a whole SEVEN notes. Given an issue of 100,000, albeit with alot of circulation, I would hope that a few more examples survived and they are out there somewhere. The other denominations and series appear to have even less examples!!

A sequence would be:-

00001-20000 ?
12001-14000 UGT
24001-26000 ?
36001-38000 UGT
48001-50000 UGT
60001-62000 ?
72001-74000 ?
84001-86000 UGT
96001-98000 ITE (vert)?

I would guess/predict notes with numbers in the missing blocks also have UGT as the control letters. For comparison 58-60K, 70-72K, 82-84K and 94-96K are all MA-A so the earlier blocks 10-12K, 22-24K, 34-36K and 46-48K may also be MA-A or at least all be the same. Could the apparent blocks of 2000 notes correspond to how they were shipped from the printer? there are examples where this happened rather than just consecutive numbers making up a shipment - another security measure? or it could all be rubbish.

How to crack the code? In the dos caritos 1 peso series some control letters are VILL-A, SCO-N, NCI-M, and FRA-L. So this would be FRA-NCI-SCO-VILL-A otherwise known as Pancho Villa - the only person I was familiar with before looking into the history of the period. In the 5 pesos dos caritos series some of the control letters seem to be the initials of the leading insurgents at the time F-V (the infamous Francisco Villa again), J-E-R (José E. Rodríguez) E-A-B (Eugenio Aguirre Benavides) etc, etc.

So names, initials, places etc seem to be an option - is L-AR referring to Leon (Domingo) Arrieta the leading revolutionary and short-lived Governer of Durango and is MA-A his brother Mariano Arrieta??

I am so glad that things are now electronic with 'saved searches' so it is now a waiting game and maybe a few more gaps will be filled in over the next 30 years or so.

I'm finding all of this way more interesting than I should, I may have to pick up a Latin American note in the near future. I never knew that the control letters were associated wither persons and places, fascinating! I'll keep an eye out for these and if I see any I'll let you know.