First; The proof marks are from the London proof house.
A; Preliminary black powder proof. This was done to the blank tube prior to much work being done.
Proof mark; This is the "View" mark applied after the final proof firing to indicate the inspector had examined it & assured it had passed proof.
B; Bore dia at time of proof, this was done in steps. In this case 13 indicates an undersize bore of from .710" to .716". nominal 12ga = .729"
C: Final black powder proof. This is the interlaced GP for "Gunmakers Proof", the London mark.
D; Nominal shell for which gun is chambered, thus 12-chamber.
Choke; Normally means that barrel was choked to at least as much as .008" (.2mm) but exact amount unspecified.
These last two ie chamber in diamond & the word "Choke, indicate man'f after 1887.
