ASM stands for Alan Stewart MaKay. Alan is the "A" in A & F MaKay audio limited in the UK that is the source of all white market Oktava microphones outside of Russia.
Occasionally some microphones take other routes out of the factory and therefore miss the ASM marking. A & F audio limited says (no surprise there) that the ASM marked ones are tested more thoroughly than the unmarked ones, but that's not really relevant if you get them from the Sound Room as they get thoroughly re-tested there in any case.
If I was buying of Ebay or Guitar Center, I would shy away from unmarked ones though, just to be sure.