Nice find, GBenson!
In addition to the above comments, with which I fully agree, my first choice for the rifle would be to keep it as is with judicious stabilization but no restoration attempts. I think they might mess up Hawk’s fine work and mislead future students as to how Nicolas made his beautiful guns. He had many unique features he used to construct these swivel rifles, and they should be preserved. If it is ever restored, it should be done by somebody who understands and sticks to Hawk’s original methods.
Hawk did not solder his barrels together, although I know of two of his rifles that were soldered together as secondary reconstructions. He used opposing dovetails on the meeting barrel flats, and held them together with elegant “bow tie” wedges capped on one end with a button-like head. The brass ribs have a bow tie hole on one side surrounded by oval engraving to surround the head of the wedge. Opposite to that is a round hole in the other brass rib for a punch to drive out the wedge while the ribs remain riveted together like rails once the barrels are removed. Your ribs show those holes.
The iron rivet you mention is really brass in the specimens I have seen, and the ribs have several holding them together to each other permanently after the barrels are removed.
If you remove the barrels from the front swivel plate, the vertical screw through the plate holding the breech plugs needs to be removed first to avoid damage.
Note that the tumbler has no internal bridle. Although this is commonly thought of as a very early feature in locks, it is also considered a later cheap alternative used in poor quality locks. I think Hawk used this method to narrow his delicate profile on his guns. Dispensing with the bridle narrows the action by at least 1/8”.
His swivel release was unique, as well, with the entire trigger guard sliding backward to unlatch the locking spring lugs.
If attempts at restoration are made, make sure they are done by carefully following Nicolas Hawks original methods. Otherwise the result will be a 21st century cobbled together fantasy gun instead of an accurate and valuable piece of history.
Nice find! Bill Paton, Anchorage, Alaska (As JTR stated, It’s a bit lonely up here on the North coast, too ;-)