Friend, you have asked a really good question. My own SVT is a 1940, has just a 3-digit number, although it also has two letters (both Cyrillic, of course, and so not what they look vaguely like in the Roman alphabet). Does your rifle also have letters after the s/n? Or is it nekkid?
I do have a couple of rifles with 2-digit s/ns, a P-14 which is s/n 305 from Winchester and a PH which is 0019; both of these are extremely early in production. And I know where there is a Mauser which is 2 digits also, this from a turn-of-the-last-century South American contact. It is a first-day-of-production rifle. My P-14 is a 'spec' rifle: one of the first made by Winchester and sent to England for inspection before full production started in the USA. And my Armaguerra 39 is only 2 digits as well: 38...... but they made so few of those that it wasn't even funny.
But MNs...... there already had been close to 20 million built by the time Adolf decided to invade, and wartime production practices in Russia bordered somewhere between desperation and barbarity. I really don't think they had time to come up with secret serial numbers, no matter how much the NKVD would have liked to. Any that I have seen are numbered up to 4 digits, with the letter code afterwards. The letter code is important and it is a valid part of the serial number, and this also holds for SKSs and SVTs. That our Govrnment doesn't even want to know about these things simply shows how utterly flawed the Registry really is.
Letter codes are also used on Lee-Enfields, Rosses, Moisin-Nagants, Mausers, Lugers, P-38s and a whole raft of other firearms. In each case the letter code is significant, important and utterly ignored by the Registry. This is why it is possible to put half a dozen Lugers on the table, all made in different plants and in different years, and all with the 'same' serial number.
Check your rifle for a letter code.
And be sure to have fun with it!
Hope this helps.
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