If you look at a lot of vehicular licence plates, you'll see a combination of letters and numbers.
I believe it's simply an arbitrary arrangement where they use the letters to signify a particular production run, and the numbers are the individual units within that run.
My best guess...
Thanks, Horilka.
I figured as much.
So that would make it an October build then, eh?
First letter indicates the month of production, second means batch "number" withing same month, then goes serial 1 to 9999, after 9999 second letter changes to next one.
But seriously if I have this right, I believe this is an I series letter gun made in Russia. The I indicates it was manufactured in 1958.
http://www.yooperj.com/SKS-24.htm
It may have a mismatched dust cover that led you to believe it's a 1950's if it's a refurb.