B6406 is a very strange engine number for any Villiers engine. As M3bobby says, a WD ML engine number starts with AAA but civilian models are different. The number should start with a 3 figure number. for example, 539/****. B6406 sounds more like a casting number than an engine number. Some postwar 9Ds had the engine number stamped on a brass plate near the gear change and this can fall off leaving just the two mounting holes. Is there anything like this?
Cheers,