I have yet to figure out the reason for the existence of the "black" prefix and serial numbers on some 1,000 Karbovantsiv notes of 1918-21, and as such I've been collecting data on the subject in the form of additional notes with the "black" prefix and serial number variety/error/damage.  Not that I needed a reason to collect more notes.  In my current mania I've managed to acquire several more examples of this phenomenon and I think I'm starting to lean heavily towards the existence of these being either due to environmental causes/improper storage of the note (ink oxidization), improper mixing/application of the ink used or contaminated ink or mid-printing ink change, and/or intentionally done after printing/an altered note. The end answer can be anyone or a combination of all three, I just don't know at this point, but I am getting closer to an answer....maybe. 

Among my more recent acquisitions were two more 1000 Karbovantsiv notes with the prefix АГ and serial number in "black" ink. This would bring my total notes with the same prefix in "black" ink to five, but more importantly one of the notes was printed on connected/linked-stars watermarked paper! My second, and both cherry-picked from sellers not listing the watermark.  Anyway, on a side note, the new АГ prefix note is only, sequentially, 794 notes away from my other connected/linked-star watermark note. Assuming that they were printed in the same run, notes of this prefix were printed at all three sites but only Kamianets-Podilsky is listed as printing on the connected/linked-stars watermarked paper, this would mean that I'm narrowing the number range in which these were produced. My first note with connected/linked-star watermark had prefix and serial number АГ 802463, my latest note is АГ 803257, meaning that, possibly, only notes of АГ 802463 or higher were printed on the connected/linked-stars watermarked paper. If you like making assumptions and using nice round numbers like me, I would make the argument that sharp-eyed collectors out there should be looking for notes with the prefix АГ and a serial number above 799999 in hopes of acquiring a watermark of this type.  My next closest note with АГ prefix is 691175, which has wavy lines or Rhine-waves as a watermark. 

Ok, back to the "black" ink. The other intriguing thing about these two notes, with the connected/linked-stars watermarks, is that the prefix and serial numbers were (presumably) applied using two different inks despite a miniscule gap of 794 between the numbers and both being from the same print run (presumably).  This relatively small gap has me very suspicious as, if accurate, the color shift would have happened mid-printing at the same facility/location. This being based on the paper having identical watermarks and all other notes with the АГ prefix, printed at Kyiv and Warsaw, having different qualities such as Rhine-wave watermarked paper, or no watermark, or lines of varnish, etc., in addition to using information available in current references. 

There are some possible font differences, especially with the prefix on the upper-left and lower-left images in the above but it's hard to tell if the font is different or if there was an over or new application of ink to the type as there is evidence of over inking or a new application of ink with the "black" prefix and serial number note, with excess ink visible around the prefix and serial number on that note. I'm leaning towards excess ink as a possible cause for the appearance of possible variations, which makes sense to me as the red prefix and serial number seems to be lacking in ink with narrowing of lines, gaps, and missing ink in spots.

For clarification, I now have five of these notes with prefix АГ and serial number in "black" ink. Serial numbers АГ 183743, АГ 263394, АГ 400451 & АГ 513625 with Rhine-wave watermark (Kyiv notes) and АГ 803257 with connected/linked-stars watermark (Kamianets-Podilsky note). 

As mentioned, I've picked a few of these notes with "black" ink and serial numbers, well, the next two notes are going to throw a wrench into the mix.  A 1000 Karbovantsiv note with prefix and serial number АВ 404371 in "black" ink and another with prefix and serial number AE 004043 in "black" ink/partial "black" ink. I have never seen or heard of the "black" ink variety being present anywhere other than on 1000 Karbovantsiv notes with the prefix АГ, and in addition the AB and AE notes are both associated with different printing facilities/locations. The AB being commonly accepted as a Kyiv note and the AE a Warsaw note. 

АВ 404371 note below, with prefix and serial numbers enlarged. Attributed to Kyiv as printing location, Rhine-waves watermark, Brown back, AB prefix.

AE 004043 note below, with prefix and serial numbers enlarged. Attributed to Warsaw as printing location, zig-zag lines of varnish (front), "W" on front of note, Brown Back, no watermark.

The AE prefix note is particularly interesting as the right prefix and serial number display blotches of "black" ink on all digits and letters, with red being the primary color and the left prefix and serial number being predominately "black".

Also, one thing that I overlooked in previous Journal entries is that while the ink for the serial number is "black" the overprinted seal on all of these notes is red, as it is throughout the entire issue of these notes. While the seal and prefix/serial numbers may not have been applied at the same time, they would have both been applied after the printing of the main designs, as there is no design for the seal on the lithography stones (see journal entry I've Been Everywhere, Man (Part1)), and in quick succession to one another. Yet only the prefix and serial number have this "black" ink.  Of course the seal could have quite literally been stamped by hand or printed by an entirely different process to how the prefix and serial numbers were applied. Meaning a different ink or mix could have been used for the seal.

I also have questions as to the color of some of the "black" ink notes as it appears to vary. The АГ 183743 has more of a purple tint to it than higher numbered notes in the same sequence, does this indicate a recent changeover from red to "black" ink? As the next three notes, АГ 263394, АГ 400451 & АГ 513625, are all much darker in color, some closer to coal black. There is little doubt that АГ 803257 could have been a recent changeover as well, with it also having a purple tint and a previously numbered note, with only a 794 gap between numbers, being all red. The same question arises with note AE 004043 as well, more of a purple tint and a mix of red and "black". Changes on the fly?

While there is still no definitive answer or explanation for the "black" prefix and serial numbers, nor has anything been ruled out, I feel like I'm closer to a solution than I was previously. Fortunately, there is more work to be done, more gathering of information and notes,  plus more speculation on my part.  More to come when available....

If you made it all the way through my, probably, incoherent ramblings congrats! I barely finished it myself,  but if you feel like bashing your head up against a brick wall some more, or are a glutton for punishment, here are some more of my Journal entries on these notes.