When I started collecting coins I was drawn to the discovery of new varieties as I found this fascinating, more so as it turned out that the standard reference books for world issues don't usually contain much, if any, information concerning such details. It transpires that the situation is much the same for medals.
So, having recently added banknotes to my collection it rapidly became apparent that 'paper' was no different and records concerning Series, control letters, dates, signatures etc was surprisingly difficult to come by for some issues. (See my Journal posts on some Mexico Revolutionary issues 'A pile of tatty notes' which highlighted control letters on Durango notes and 'It's made of paper' which mentioned the Chihuahua dos caritas series).
I have also acquired quite a few Latin American notes beyond Mexico as the fortunes of individual countries are intimately linked to those of the wider region. Compared to the intricate designs and vignettes of the early Banco issues I was attracted to the simplicity of this readily available 1 colon note from El Salvador- and not just that it had an extra date on the back (17-Aug-1973 in this case).
So for a single Pick number (115a) it appears there are different signature combinations, front and back, and multiple series, dates etc with no mention of what these are in the SCWPM issue that I have and El Salvador is one of the several Latin American countries missing from the 2014 Banknote Book volumes. Numista records 24 different dates but these are not linked to the Series letters so this prompted a quick look at auction records etc and it was great that the sequence of Series letters used rapidly became apparent with the serial numbers suggesting 1 million notes per Series for 1972-1977.
GN - GW
QA - QJ
HQ - HZ
PK - PT
IA - IJ
UB - UK
EF - EO
RK and RL
However the examples that I could find were still very limited (26 out of the possible 72) and the situation with the linked issue dates is even worse. Some Series have several issue dates so I wouldn't be surprised if the number of dates was nearer to 100!
This collecting lark is a very slippery slope as I now have to look at all the other 1 colon issues and also the other denominations. Fortunately this can all be worked out electronically as auction lots now have pictures, and it keeps my wife happy as she says it keeps me out of trouble - it doesn't .