Some additional information.
Now that I have the tang in hand, the hook will start in the tang under pressure, it's that close. Do you think they are made to be tapped on the first time to ensure a tight fit? I do believe I could place themuzzle on a piece of leather on the concrete shop floor and tap the tang on with a hammer and piece of oak. Concern would be getting it back off. That may be easy given there are only two bearing surfaces owing to the taper of the hook. Like you,I have to believe the hook is centered on the barrel, suggesting the visual difference between the alignment of the hooks of the two barrels laid in the channel may have more to do with the slight difference in hook width and slight extra room in the barrel channel. Don't have the gun with me but something to check.
if you are that close and still have the 54 fit..
I'm not sure what you mean by this (54 fit)
just remove material on the nipple side of the tang on the hook inlet.
Uh, the nipple needs to go left (from above) so material needs to come off the left (from above) interior of the tang, not "nipple side", correct? I'm fairly inclined to tap the tang on at this point. Let me know about that.
It's all close enough that I do think it can work.I may have to shave the tang face a hair and insert a shim between the barrel and lock.I have a tough time thinking of that shim as "craftsmanship" and would like the tang to continue to function with the 54 cal barrel. Challenges........