Actually that's BM144, the Vb flown by Zumbach before he got hold of EN951, which is the one I'm building. You can tell from the different "duck" artwork and the (fewer) kill markings. the mounts are quite different on the two aircraft - the BM144 one is markedly more "triangular", pinched at the top, whereas the EN951 had two parallel bars. There may also have been a short "brace" at the front of the base but that would've been a tiny piece of 5 amp fuse wire too far.
(Either way you're probably right that the mirror was flatter, just that I used the little transparency and coloured it with silver Sharpie on the back before painting in an attempt to make it look "reflective")
Edit: correcting myself here: the second of your pics is BM144 (Zumbach's first Vb), the first one is EP594 (his second). Again, note difference in the ducks - baseball bat up, baseball bat down. The mirror mounts look so similar on the two that I'd almost bet it was taken off BM144 and put on EP594 when he switched planes. For EN951 he had a whole new mount built. I wonder what the reason for the extension was anyway?
It can be noticed that the Zumbach's mirror support is attached to the orginal mirror base and its bracket with a clamping screw. It was done for all Zumbach's Spitfire Vbs in the same way, with the same parallel, slightly bent matal bars to move the standard mirror more up. The different appearance of the mount(s) is a result of taking pictures with a different angle.
IMHO the standard mount didn't give enough of the panorama back view especially down because it was too low attached. The rear view was limited by both the fuselage top, the antenna mast and the tail while the attack was from the back and above, also down. Having the wider picture a pilot could react quicker.
the standard mount ...
EN951 ...
BM144 ...
the source: the net.
