Quote Originally Posted by tacfoley View Post
There has never been a .60 smoothbore gun in British service. No unit would have requested a different bore or calibre arm than the rest, except the Light Division who were equipped with the Baker rifle in the late 1790's and early 1800s - and they were flintlocks.

David Minshall notes -

A government rifle will NOT carry the commercial marks of the London or Birmingham Gun Barrel Proof Houses with their usual marks and double 25 bore size marks. However these can sometimes be found IN ADDITION to the defaced original government proofs showing that the rifle has been correctly (and as required by law) submitted for proof prior to civilian sale.

This piece is a muzzle-loading antique - not a Section 58 which refers only to obsolete calibre cartridge-firing small arms.
That is odd since I have seen many guns sell as Section 58 that were muzzle loaders and did not fire cartridges.