The 9B means that the barrel is 9 bore and would have measured .803 to .813 at 9" from the breech face at time of proof. If it is now greater than .813 it will be deemed to be out of proof. The 10M will indicate that the gun is choked to around .775 (10 bore) at the muzzle. The 'Not Fot Ball' is indicative of choked bore guns produced under the 1875 Rules of Proof and dates it between 1875 and 1887. It would have been proofed with a load of 12 drams black powder and 3 ounce of shot. (Note this was the Proof test load). The service charge would be 6 drams (164 grains) powder and 2 1/4 ounce of shot.
Hammerless 8 bores are not all that common and ejectors even more so. Quite a few of these old 8's, if in good order, will re-proof for nitro quite successfully. Use coarse grain black powder as you will need a slower burning rate. Lagopus.....
