Sorry, don't know what the "M" stands for but someone on this forum will. I just ordered a 527 carbine in 223 that should be here next week. I'm glad to know that the rings come with the rifle because it's difficult to find other rings that fit correctly. I have a 527 varmint in 204 and the CZ rings are the only ones that seem to fit well. I've killed a lot of coyotes with my 204. I'm hoping the 223 will work on hogs.
Good luck with your new rifle.