Doesn't represent anything, its half of the engine suffix code. All 1st gen Camaros (and most GM cars from the early 60 thru 1970) have two letter suffix codes indicating what the engine is. DZ is a 69 302 Z28 engine, MK is a 327/275HP L30 with a manual transmission, HA and HB were the two assembly codes for the 69 L48 350/300HP with either a manual or automatic transmission. Theres literally hundreds of 2 letter engine assembly suffix codes used by GM over the years up to 1970 when they went to 3 letter codes. There were single letter engine suffix codes used in the 50's and maybe into the early 60's but I would bet that you M infront of the K is probably just filled with paint or so lightly stamped you can't see it.