This is interesting. What's the name of the book & author Tony? The dual Cyrillic prefix system was used long before the SKS as MNs sport them all over the place.
One thing that has always bothered me is that the Russian SKSs don't seem to follow the European pattern.
Take for example Romanian guns with dual letter prefixes, they are in a somewhat logical progression. Take 1958 for example:
BC then CD then DE then EF (then FE unfortunately,^%*%$^ Romanians!)
Cyrillic prefixes seem to be all over the place regardless of year with very very few 'repeats'. They could have gone purely alphabetically such as: AA, AБ, AB, AГ, AД, AЕ, but that sure doesn't seem to the be the case:
1949 prefixes in Notya's survey & in my photos: АГ, АУ, ГМ, ЕБ, ЕМ, ЖБ, ИЧ, ЛГ, ЛМ, MB, ОВ, РГ, РЖ, РМ, РН, РО, УК
I think it's a pretty good stretch to say the SKS45 is numbered alphabetically and XX = the 1234567th gun produced. I'm convinced that they had some kind of code they used to increment both the first and second letters. I'm really going to have to get off my tail and start looking into it in more detail.