If you look closely you will notice the graduations are not the same distance apart between the M and the standard drum.
I have them displayed flat in a file and see if I can show the difference.
Doing that was an interesting exercise with the drum on a mandrels, the camera on the carriage of the lathe and bull gear on the lathe rotated a few teeth each shot.
Lots of fun and the things we do playing with Lee Enfields and scopes.
The ranges due to the tooth difference using the .303 plates and 7.62 are not the same but lose enough.
There is an issue chart showing the correction factors and I have one here...someplace.
Going to get some charts made up ..someday :-)##