Eric, you describe it much better than I. What you described is exactly the type of patchbox hinge that this gun has. The gun was brought to California sometime before the gold rush. It had already been converted to percussion, quite likely by the original builder, since the workmanship on the conversion is impeccable. The original owners found it almost worthless in the west I suspect since it only .45 caliber, and smoothbored, to boot. In any event the guns primary damage appears to be from being kicked around in closets and attics for a couple of hundred years. Its major damage is to the forearm, and stems from the underlugs being very thin folded brass, in very shallow dovetails, which separated when the forearm shrank with time.
Hungry Horse