always safety first John
but i'm curious - i have a light unaltered 12 with the bores proofed at 13 - i have seen one marked 14-
if 12's can have tight bores- why not a 10- and would a 12 have originally had an oversized tube marked 11 in that day?
i think the weight also speaks to an original 10
but - having said that - i'd be measuring the walls ahead of the chambers very carefully -
long chambers were made as shown here
look about 1/2 way down in the second column from the left to the specs for Dr Willimson's new Lefever - a 12 ga with 3 1/4 chambers for a heavy load
http://library.la84.org/SportsLibrar.../SL2604014.pdf