By 1950 all Vincents would have been fitted with the 6 1/2" Miller unit unless an 8" was found in a dusty corner.
I have borrowed (and hope to buy) a 7" Miller which I think originated on a Velocette. It's a little deeper than the exact correct 7" Miller which some Vins got. Will it give clearance problems with an AVO front damper? These are fatter than a Vincent damperSome Export had 7" some 6.5" (Gidraulics)