Pooch;
I am operating from memory now, but my recollection is the chamber mark (12 in Diamond) was not used prior to 1887. Along with the chamber marking there should be a mark for bore diameter when proofed. Prior to 1887 this would be in whole numbers only as 13, 12, 11 etc. After 1887 it could have the "Tween" sizes as 13/1, 12/1 etc.
You do not mention either "Not For Ball" or "Choke" so it may have been cylinder bored in both barrels.
Without the NP mark it is a black powder proofed gun.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra