It's the Cyrillic letter found AFTER the 4-digit serial number. Letter series guns we're made in 1955 and 1956 (and some 1954 Izhevsk's). Many collectors believe that the "К" series (1956) are the very last SKS's produced in the former Soviet Union, just before the machinery was sold and relocated to China. Personally, I believe the jury is still out on what suffix letter designation was the last, but so far, "К" seems to be the last.
I've seen a 1954 Izhevsk with a Cyrillic suffix "X". That is the furthest down the 33 letter Cyrillic list so far, but because it was made in 1954, it is not the last produced.
The SKS Registry, with 192 responses so far, contains the following information regarding the 1955/56 "letter series" Tula, single letter suffix:
Д -- 10 occurrences
И -- 15 occurrences
К -- 6 occurrences
Again, "К" does appear to be the last !!