M Series Identification

M SERIES	496cc 82mm x 94mm SV M20, 499cc 85mm x 88mm OHV M33, 591cc 82mm x 112mm SV M21

1945	M20 as WD M20 girders, rigid frame, saddle, no footrest rubbers.
1946	M21 introduced as M20 but with larger engine.
1947	Footrest rubbers from XM20.7390 frame.
1948	M33 introduced as M20 but with B33 motor, engine breather moved to timing cover, speedometer mounted centrally on front forks.
June 1948	Telescopic forks fitted frame modified to take account of wheel clearance.
1949	modified gearbox, larger dynamo.
1951	Plunger frame option, sidecar lugs fitted to both sides of machine, light alloy cylinder head for M20, M21.
1952	Dual seat option.
1953	Headlamp cowl incorporates speedometer, ammeter and switch, boxed in rear number plate with stop/tail light and reflector, under-slung pilot light.
1954	New plastic tank badge.
September 1954	Pilot light in reflector, steering head lock, ignition control changed to pull to retard, monobloc carburettor.
November 1955	BM20.4125 engine, BM20S.8000 frame M20 discontinued, M33 rigid discontinued. M33/21 spring frame standard, 8" front brake, valanced front mudguard.
August 1957	BM33.2808 engine, BM20S.10045 frame, M33 discontinued.
1958	M21 no headlamp cowl, return to separate shell and small panel for light switch and ammeter.
1959	M21 plunger frame only but with some rigid frames from stock.
1960	M21 rigid discontinued.
1961	M21 alternator added to machines supplied to the AA.
1963	M21 discontinued.