Cyrillic numerals
A - 1
B - 2
Г - 3
Д - 4
Є - 5
S - 6
З - 7
И - 8
Ѳ - 9
I - 10
K - 20
Л - 30
M - 40
H - 50
Ѯ - 60
O - 70
П - 80
Ч - 90
P - 100
C - 200
T - 300
Ѹ - 400
Ф - 500
X - 600
Ѱ - 700
Ѡ - 800
Ц - 900
There are quite a few letters in the above list that are not part of Russian alphabet, and weren't for close to 100 years (Major reform of the Russian language took place in 1918, when a bunch of letters were depreciated and some were pulled from use entirely). To me the above looks like partial Cyrillic alphabet used in old (primarily religious) books from 1860s. The above list is probably not that useful as it also misses some of the letters currently in use.
Russian alphabet has 33 letters, 6 more then English. Two of the letters are silent, and soften or harden the preceding sound. One of the letters Ё (yo) is depreciated, and commonly substituted by Е (ye) when written (but not when pronounced, and the speaker is expected to be able to recognize the case when Е is written, yet Ё should be pronounced.
A number of Russian letters do not have a single letter English analog and generally are transliterated using multiple English letters (ё - jo, ж - zh, ч - ch, ш - sh, щ - sch, э - je, ю - ju, я - ya). At the same time V and W would be transliterated as В if transcribing something phonetically from English to Russian.
So this is the current Russian alphabet:
А (ah)
Б (beh)
В (veh)
Г (geh)
Д (deh)
Е (je)
Ё (jo)
Ж (zhe)
З (ze)
И (yi, as ee in see)
Й (yij, as y in yes)
К (keh)
Л (leh)
М (meh)
Н (neh)
О (oh)
П (peh)
Р (reh)
С (seh)
Т (teh)
У (you)
Ф (ef)
Х (kha)
Ц (ceh)
Ч (che)
Ш (shah)
Щ (schah)
Ъ (hardens preceding sound, silent on its own)
Ы (yui, like i in still)
Ь (softens preceding sound, like ~ acting on n in Spanish ñ, silent on it's own)
Э (je)
Ю (ju)
Я (ya)
13 out of 33 letters have look-alike Latin analogs, and are most likely used in the CFC database:
А as A
В as B
Е as E
Ё as E
З as #3
К as K
М as M
Н as H
О as O
Р as P
С as C
Т as T
Х as X
In addition У might be transcribed as Y depending on who registers the rifle.
The rest lead to confusion.
For sure you might have an SKS with the serial number fully registered, I have one myself. But my other SKS only has one letter out of the two in the database, and the other letter is untranscribable and is dropped. Latter seems to be more common.