Thank you Raimey and Greg!

So in this A. Lancaster's 120 years of use, the bores have been moderately lapped out as the chokes now measure .005 and .010. I should go back and re-measure the bores but when I purchased the gun two years ago they measured 0.731.

Interestingly, when it was nitro proofed a new bore dimension (for that proofing) was not stamped on the flats. I guess the proof house accepted 12B measurement as an accurate dimension.

Dr. P