Not a lot of detail in the relevent photos, but it looks like the new breech block was silver soldered onto the top of the breech balls. The photo of the right side appears to show a relatively short length of joint.

I have tried some sketchy things with guns in my time, but even with the mild chambering, I would have some trepidition with trusting who knows who that did the work who knows when. I bet there were absolutely no calcs performed except "I think that'll work".