M is for Modified.
regarding the slamfire issue
don't know about 3-4 years ago and the used condition you purchased your's in .but when they hit the states prior to 89 they did have issues and the US distributors were replacing any sks's that had the issue. i had one and after 1/2 a mag it started dumping 2-3 round burst. neat but not worth the hassle. dealer took it back and gave me a nice milled norinco ak in its place!