Introduced in 1931 to supplement the Colt range of 250cc machines designated LB, LF & LG (B=SV, F&G=OHV), the 350cc M series shared the sloper style of the smaller engines but with bore & stroke of 72x85mm.
Quite an advanced engine for its day, it sported fully enclosed valves with pressure lubrication via the dual-plunger oil pump, and primary chain in a cast aluminium case driving a 3-speed Burman box. The chaincase made the engine far less prone to oil leakage.

Courtesy New Zealand Classic Motorcycles

Ariel Motorcycles of the 1930s