It seems strange to have 1 barrel .729 and the other .719 but I have a theory, based on having owned several Britt boxlock waterfowlers. At the time some of these guns were made shells with felt wads were used and the gunmaker wanted to insure combustion and perhaps increase performance by having tighter constriction in the barrel.
Given the manufacture date of - I think - 1963, do you think that's still a possibility?
Felt wads were never passé in Europe. The last pallet of Eley 16 gauge I bought was felt wads, VIP was the load, if I remember correctly. Purchased early 2000s.
‘Course, they might be, now. But 1963, no way.
Best,
Ted
