Heres what I know. the K6 denotes the number of ribs. (6) the rest of the number SHOULD be the length in inches. That is xxinches & x tenths of an inch. so a 922 should be 92 & 2 tenths, a 923 should be 92 & 3 tenths inches, and a 950 should be 95 inches and 0 tenths long.
Some parts stores only want to stock certain #'s so they will round it up/down to suit what they carry.
One thing to do is 'burp' the engine and see if you can get the belt to stop in the place where the numbers are on the belt. Somewhere around the outside is a molded in/on tag that has the part number on it.
As long as you have enough adjustment on your adjuster, the exact length is not super critical. Personally, I'd go with one of the 2 that are closer (922 or 923) than the other one, especially if I could not read the number off the belt on the car.
There are two ways to do the numbering system like sean883 is saying:
060950 would equal a 6k950. Just get rid of the "K" and add a leading 0 to the number of ribs and to the length.