Great info. Thanks to all that have responded. I don't have the gun here to check the bores for the original proof size but will try to find out.
Pete- What would you be looking for at the thin spot? I understand that it is about an inch or so long in a isolated spot on the left side of the left bore.

I had never even considered using black powder loads. I have done a fair amount of reloading with nitro, including PB, Longshot, Int. Clays. All of the loads being at 8000PSI or less. Forgive my ignorance but can you handload black powder in modern cases? There is no chance, near as I can tell, in finding commercial black powder loads in Canada. Commercial nitro loads in 2.5" are hard enough.