Congrats on a great acquisition, Ed. The priming pan was no doubt mechanically fitted via a dovetail joint in that groove, the extensive corrosion over centuries has eaten away at the undercutting. If you merely want to display the gun and not shoot it,
you can probably attach your replacement with epoxy. Do the aging on your replacement pan and cover assembly first, and you can mix various organic materials like sawdust, powdered charcoal, and even some ground up rust in your adhesive to make it look like the thing has been together for donkey years.
The barrel is interesting for not only the pan attachment but for the lack of an extended tang on the breechplug. Having restored a lot of Oriental guns (Indian, Tibetan, Chinese, and early Ottoman) I have seen exactly the same format and can confirm the dovetail attachment in all of those instances.
If you have Robert Held's
The Age of Firearms (1957, and several subsequent eds. incl paperback and foreign languages), Fig. 39 on p.28 has an excellent 3/4 view of a lock and pan. You will note that the pan, being essentially square with the cover pivot screw on the outer rear corner, has a round bowl shaped depression with the touchhole drilled BELOW THE RIM on the side adjacent to the barrel, and connecting through to the bore. It is not the oblong or bidet-shaped depression with a trough-like path to the touchhole as is used on later lock systems.
The deep vertical notch on your barrel is to help retain a flash shield fashioned from a thin iron plate, running from the top flat of the barrel to a tubular sleeve that supports the long pan pivot screw. The drawing in Held's book says it all.