Providing there are no further proof marks since the pre 1887
not for ball then your gun remains in proof.Marked as 12b/14 means it could have been up to .740" at time of Proof. 1954 General Rules of Proof state:-
#7 ( Barrels enlarged in the bore) unproved if 9"/diameter....."is .010 or more of an inch greater than the nominal diameter marked on the occasion of the last proof or re-proof."

Thus .740 t0 .749 in proof, .750" out of proof (only if latest proof is 1887)
Hope this helps, I'm still betting on WR as action manufacturer

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745