Quite obviously a 12 gauge chamber will not accept a 10 gauge shell. As it has been determined this gun accepts & fires a 10 gauge shell, it does not currently have a 12 gauge chamber. With a current barrel weight of 4lb, 9.6oz that would have been one heavy 12 gauge.
With the 12 bore stamp & post 1887 marks that means it would not have accepted a 12/1 (.740") plug gage at time of proof. What do the bores measure now, were that also enlarged or just the chamber.
If the bores have also been enlarged it would have weighed even more as a 12 or perhaps it did simply get mismarked at the proof house. A firing pin spacing of 1.131 does seem rather close for a barrel set of this weight regardless of gauge.
