Well, back from holidays and another Villiers question to answer. JustinF has given you the basic info all I can do is clarify things a little more. Little is known of exact Excelsior frame and engine numbers up to 1954 or thereabouts. possibly the VMCC marque specialist could tell you but its really not a great issue as like most British firms of the day Excelsior sold the same basic bike but installed either their own or Villiers engines of different capacities and gave it a different model name. In its day your bike was sold with the 122cc 10D Villiers engine, Excelsiors own 148cc engine or the 197cc 6E Villiers fitted. The only other difference lay in the quality of electric lighting fitted. The cheap model had direct lighting (brightened and dimmed as engine revs rose and fell) or rectified lighting with a battery that ensured both a consistent lighting level plus you could fit a stop light on the brakes.
As far as the engine number is concerned don't read too much into the fact that it has a 10D engine originally fitted to another make of bike. Old Villiers stuff was so cheap for so many years that it was cheaper to replace an engine with another second hand engine from the wreckers rather than spend good money on rebuilding a worn out one. For that matter even if you went to a dealers and had a reconditioned exchange engine fitted there was no guarantee your replacement would have a appropriate engine number for your make of bike. Broadly speaking all 10D engines are the same, it doesn't matter what make of bike it was fitted in. A 10D engine is a 10D engine, the make of bike is pretty irrelevant.
Anyway, your bike looks very original except for the paint job of course, and should give you lots of fun.
You can get copies of all the necessary technical material, manuals, transfers, catalogues, etc from the english Vintage Motor Cycle Club. They have a website
www.vmcc.net and are very helpful.
Cheers,
