I've had 3 of these cameras, bought for resale as they are premium priced usually as a 'collector's item'. Only real difference with the normal M2 was the first Leica with the quick-load film system later introduced as standard on the M4. The camera also had the wiring for a Leica motor (no film diagram area contacts though) and was protected a bit better from dirt with some seals.
Story... My brother was given a M2 KS-15 in Vietnam to use. Didn't think anything of it at the time as I was too young and not into photography, but years later when I had an M4 he mentioned the same film loading system in his camera. The M2 KS-15 I found were all in Thailand, I guess left behind.