No assumptions necessary. We already know Izhmash did not start sequential serial numbering of rifles until a few years after AK74 production began.Cool kit! I still haven't posted my '74. I believe it was earlier because mine didn't have the feed ramp and has the <PB> instead of the <П>, so that's a couple changeovers we might have nailed down. My s/n is 806311, yet it has earlier features than the 287774. So it looks like we can't make assumptions expecting serial numbers to be sequential.
For one thing, and maybe I'm thinking of Tulas, even after they changed from Cyrillic prefixes and 4-digit numbers to 6-digit numbers, they still stamped the first two digits (prefix) a little bit separated from the 4-digit component of the serial number. This isn't my thought since I think I've seen it somewhere else before, but I believe the two digits are really just a prefix in number form instead of characters and not necessarily sequential. I'd assume Izhmash did the same until they transitioned to sequential serial numbering during AK-74 production. Take this '75 Tula bolt carrier for example:
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The numerical prefix on the '75 Tula is even more offset than the letter prefix on this '61 Tula:
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There is also the complication of multiple assembly lines. Did each line have its own prefix? Or was there a prefix of the month and each line just had a different block of serial numbers? The '86 AKS-74U kits that came in were all suspected of being from the same factory time period, yet they had two distinct serial sequences. Granted, that was over 10 years later and at a different arsenal.