M21s had an alternator fitted for the last 3 years of production (1960-63, Engine prefix BM21A )...These were 'to order only' for the Automobile Association...
They still retained the dynamo, the alternator being used to power a radio set...
The drive side mainshaft was altered to accept the alternator rotor, thus losing the engine shock absorber assembly...This was moved to a Triumph type 4 spring clutch...
The alternator stator was carried on a separate aluminium carrier mounted within suitably modified steel primary chain cases...Of course you could use the alternator B Series cases if you modify the footrest mountings on that side (I did that when I fitted alloy cases to my '51 rigid B33)...
I think I still have a crank and M21 steel cases somewhere, though not the stator carrier...
I fitted an alternator on top of the magneto a few years back in a (complicated) one off housing I machined up...It worked well for a few minutes (
) until the dynamo clutch burnt out..
...Ian
email (option): ian@wright52.plus.com