CJF;
I may not have made myself clear. Normally a British proofed gun will have the bore size marked as per the largest plug gauges which will enter the bores. Then in 1887 they began marking the chamber size. Thus it would appear this gun per its earliest marking had a 12 chamber with an 11 bore. I think that a "True Chamberless" gun with 12 gauge chambers would have had a 10 bore.

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