I always suspected that one or two of the operators of the older models would just add an "M" sticker on the side of the TU-154 sign and keep flying them in and hope we wouldn't notice
Since this *is* the Spotters' forum, I should point out that a side-view of the rear of the fuselage (jetpipe of the no.2 engine) is a recognition point for distinguishing the 'M - it has a straight, vertical face, whereas the older versions had a 'scalloped' look.
Plenty of both the 'M and 'B-2 variants still active in the Moscow area!
Rhys.