The Guy Martin Program mentioned the use of an MG mirror on N3200.

Standard fitment to the MG sports cars of the time (and many other British sports cars) would have been the Lucas type 160 mirror as replicated here and sold by many other MG spares companies. This mirror was still in use post war as it is on my fathers 1954 MGTF

http://www.vintagemgparts.com/mirror_arms.html

I don’t know how this differs from the Desmo mirror already mentioned – Desmo were also in common use by car manufacturers of the time – except to say the Lucas has a single fixing centrally mounted on the back and the Desmo mentions a two lug fixing but I can’t find any pictures of it.

Presumably the surround mounting was then made locally in the workshop at the squadron. It is similar in design to some Brooklands era race cars I’ve seen but there wouldn’t have been many of those to steal so would have been copied from pictures I guess.