IMHO the 1 oz and 2 3/4" are part of the original proof 1925-1954.
Would that then be a rebarrel, or by his son, A F Ford?
20 would be .615" - .625". 15.9 = .625 so likely honed a bit prior to London reproof last year.
Thanks Drew.
Nigel Brown places the serial number 15997 in 1931 per help from another forum.
I agree that the 1 oz. 2 3/4" proofs were original to the gun at that point. I don't think it was re-barreled, but who knows?
20 over C put the the original bore diameter at 0.615 (15.6mm) as you said. I'd bet it was honed to 15.9mm (0.626) in 2025, and sent to the London proof house for reproof, which it passed at CIP over N specs with a new 70mm chamber proof.
