This is interesting. Justin was spot on in everything he wrote and this will be an absolute first, a prewar Villiers engine with no prefix to the engine number.
Can I ask,
a) What information has been cast into the back of the cylinder on the transfer port, and
b) What diameter is the inlet manifold where the carburettor clips on?
Last thing, most of these engines were fitted with a decompression valve regardless of what it was originally fitted too. I've only ever seen it deleted on some of these engines fitted to very cheap ATCO mowers. Most mowers and other non motorcycle applications still had them fitted to assist starting and to stop the wee beasties.
These are marvellous wee engines. I've done many miles on a 2 speed Excelsior powered by one. I never knew I could have so much fun riding such a tiny little bike. It completely won me over.
Cheers,