Interestingly, I just got some PMAG's that have a newer date stamp than the M's shown above by Stick, but aren't M's. They are somewhere in-between old and M.
The mouth of the mag and feedlips look like an M, and the catch up top for the dustcover is two cuts instead of one, but the rest of the body is like the old. The follower seems somewhere in-between but shows no marking. The dustcover on mine does not snap on the bottom.
Date stamp is 2/09 so not sure what to think. I guess Magpul is still making non-M