Do they measure the bore on rifles in the same way as they do shotguns? If that is how it is then that would put it between a 410 and a 28 bore. Looking at the photograph of the muzzle and comparing it with the 16 bore shotgun barrel it looks bigger than that to me - more like a tight 20 bore. I will ask the owner if he can measure the diameter which would give us a guide. The numbers in the diamond look like 430 or 480 over a 'c'.
John