The early 1928 built 500cc TT Replica I bought that had been stored in Albert Reynolds attic for many years was original and all aspects were correct to dispatch records.
It had a BTH M2 mag with the small taper. The bike was supplied with a detachable battery and lights so these could be removed for competition and refitted to go home.
The mag is identical to those used on most Douglas machines of the period and you might have better luck looking for a Douglas mag. I sent one to USA last year. However difficult it may seem to find such components in the UK, we should imagine how isolated our overseas friends must feel when trying to find such components. Kind Regards Roger M