I'm sure you know that the voltmeter will not have been unique to Whirlwinds. Take a look around e-bay, find one that looks exactly like your photograph - and bingo. Here's something very close, 5U/1693, from Spitfire/Lancaster and many others no doubt.
PS. You'll notice the small variations below - but same Part Number. From this - I'd say you could safely use either in the fair assumption that only one AM Spec existed for 40V meters and that Whirlwinds, somewhere, could have been fitted with any variation.
..... and another.
