Three kinds of M drums. Without fins. With fins but small rim. With fins and rim same as J. All M drums have holes for three studs and three counter-sunk holes for the fixing screws.
Norbert You can get brand new ones from Oliver Richardson. They have steel rather than ali fins - but at least they will be un-skimmed and round. I got some 10 years ago via the late Bob Hudson - they have been excellent. I still have the old ones if you were desperate - they will have been skimmed xx times and are probably not round