I'd guess the pre-rifling diameter to be the "52"/"53", which would be around 0.450", but I don't think there was a 52/53. Birmingham proofs and I'd like to see the rhombus stamp a little closer. There was a "50" and then the small bore rifles began with 0.451" and then 51.05 & 54.61.

Kind Regards,

Raimey
rse