Now I don't specifically know 1951 Duggy electrics, but brake lights weren't required until circa 1954,
so its highly likely your switch was added sometime after that. ?
So could have been anything that was to hand, can you see a brand or logo on it.

These days, switches of Lucas or Miller parentage are what is commonly used.
I recently invested in some Royal Enfield switches - and they were inexpensive.
These are smaller and lighter than the originals, time will tell if they last as long.
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