In regards to
Yugo
M48s, it is not a prefix... but a batch number.
They are only present on the first 100,000 M48 rifles, then was dropped and just the numbers were given per model from that point on. The "K" indicates that rifle was in that number batch. From the alphabet in Bogdanovic's book, would seem to be the 13th batch (judging the example he made in the book).
With M48s, they did somewhere around 52,000 to 53,000 rifles in the first year of production (1950). Your serial number works out to 1951 production, which was about 92,000 rifles. There are some discrepancies in the production numbers, which is mentioned in Bogdanovic's book.
My M48A does not have a prefix... and is the 51239 M48A rifle produced. Works out to 1953 production.