Your mark under the crown appears to me to be VBP. This is a special definitive proof where the barrels were proofed only once in the finished state, but with the heavier Provisional proof charge. This proof was made available upon request of the gunmaker in 1868&* stayed in effect until I believe 1925. Birming proofed guns carried the crown over VBP while London proofed guns used a Lion Rampant over VGP. The BP & GP stood for Birmingham Proof & Gunmakers Proof respectfully, I have never to my recollection seen what the V stood for.
As this mark was not used until 1868 this would make this a fairly late gun for a muzzle-loader. I find it a bit unusual for a gun of that era not to have the hooked breech
