Rd Show-
Thanks for looking. The "at or near" 5/8ths inch bore makes the gun roughly a .62 caliber. From one source (not sure how trustworthy) it appears that the 20ga is right around .62 caliber. That the number 58 appears in an oval and the bores appear to measure about at 5/8ths is likely coincidental. The .62 bore size apparently was not one of the more common, I found a couple in my Internet search (no double guns...but obviously they are out there). One such .62 was a single barrel "stalking rifle."
The feel I get from this old gun is that it was likely not intended so much as rifle/shotgun, but more of a rifle/musket because the bores are of the same caliber, only one rifled and one smooth. The musket may have been loaded with buck and/or ball and the rifle with patched ball. The presence of sights may also tip the balance in favor of large game hunting. However, the owners appeared to have preferred or needed the musket function as that side appears to have had easily twice the use of he rifle.
This old gun may have been one that helped tame the New World. I imagine that the owner(s) were very well armed with this combination. Most men of that era would have had single shots, and would have been handicapped by slow reload in the event of a miss. The musket would have offered a speedy reload at the expense of accuracy, the rifle and accurate shot at the expense of a speedy reload. But anyone armed with this combination would have been nearly as effective as two armed men.
I was hoping the cleaned up proof mark would be the conclusive bit of information. Other than that, the "QR" stamp may have some meaning as it appears on the locks as well. The letter B appears in several places on the barrel. Maybe Bavaria, Birmingham?
Thanks again-
Doug
