OK, I'm back home now and I have dug out my unrestored original Miller Tail-light and my very old repro that I bought from Domiracer in the USA nearly 2 decades ago.
Here are the backplate measurements
(original first, Domiracer second):
Thickness: 1/16th, 1/16th.
OD: 2-1/2", 2-1/2".
ID: 1-7/64", 1-7/64".
6 hole PCD: 2-1/8", 2-1/8".
6 hole Dia: 5/32", 5/32".
2 hole PCD: 1-1/2", 1-1/2".
2 hole Dia: 3/16, 5mm.
Two things (in red) stand out:
1. The ID in the "MSVH Y3" excellent photo's above is quoted as 1-1/16". I would recommend the original 1-7/64" as this gives almost 50 thou greater clearance for the 2 x barrel-terminals on the rear - which are of a larger dia on the repro variants. Electrical short circuits not wanted here!
2. The 2 inner holes have been drilled by Domiracer to take 5mm screws, which at approx 10 thou dia bigger is of no real consequence. (Even Concours Judges wouldn't know
).
As an aside, I think the thick paper gasket on the original lamp is a previous owner's attempt to help prevent vibration blowing the tungsten filament bulbs...!
Hope this is helpful Chris,
Peter B