Hi Barry..Viewed from the drive end the M20 mag should run anti clockwise...there is an arrow marked on the drive end cover. Also viewed from that end the pick up is on the left of the body. There should be four location dowels in the base (5/16" dia.)
Viewed from the points end the adv/rtd cable should enter the body to the right of the points.
On early bodies the curved portion that the dynamo sits on runs along the whole length of the body and meets the drive end housing.
Check to ensure the base is not badly worn, the taper is good where the drive gear fits and also the armature thread at that end is not damaged.
MO1s are all the same in terms of function..there are just the physical differences for different applications, so as long as it conforms to the description above it will be OK....Ian
email (option): ian@wright52.plus.com