There is a sister gun (in my opinion only) that Shumway presented in his Muzzle Blasts article in November 1982. It has a rifle guard very similar to that on #19 but a round toe. Walnut stocked, with a Reading or maybe Lncaster sideplate. The patchbox end is brass and hinged, very simple Reading finial. The entry thimble is nearly identical. The barrel is short, 36", .67 bore and octagon to round. The buttplate is not specifically French but of a fowling piece type. Note that others don't see it as clearly related to 19 as I do. But if from the same hand or shop it indicates use of scavenged parts or at least a variety of sideplates, buttplates, etc.
Alex, I sent you a PDF in an email.