1939 bikes carried the KM24 engine and frame number prefix, again starting at 101.  
Numerous changes were made for 1939, including a new petrol tank with tank-top instrument panel which housed Lucas ammeter & light switch, the oil pressure indicator button and a Lucas mobile inspection light. A new shape oil tank was fitted, with the frame-mounted tool box below and behind it. A short side-stand was added to the nearside lower frame rail. The 8 inch Lucas Headlamp reverted to a plain shell, a valanced rear mudguard was fitted, with option of a valanced front guard. The exhaust pipe profile became more angular and the silencer lost its separate tail-pipe.
The magnesium gearbox shell was discontinued in favour of aluminium alloy, and a close ratio gearbox was offered as standard, with the option of the wider ratios of 1938.  Minor external changes were made to the cylinder head and barrel pushrod tunnel, and the engine breather was moved to the upper nearside crankcase. Internally, improvements were made to the timing gear and pushrods, an additional ball bearing was added on the timing side of the crank shaft, and minor changes were made to the carburettor settings.