I managed to tighten the frame up by carefully upsetting the metal of the lugs with a punch and hammering them back into shape and filing everything back to original appearance. The Breech face insert was missing along with the firing pin and transfer bar, or at least the upper part of it. I've got an Iver Johnson I rebuilt from a sack of rusty parts. I Googled 'u s revolver co' and came up with several hits. (Actual guns that could be called that, like the old Webley-Fosbery or the Mateba are quite uncommon.) This may have been the origin of the confusing term 'automatic revolver' often seen in mystery stories. Some of their guns were marked 'automatic', which meant automatic ejection (top break) as opposed to the manual ejection required with the swing-out cylinders of the then new Colt and S&W revolvers. In any case, they made the 'U.S.' line from about 1910 to about 1935 as a second line for sale by mail order and in stores like Sears and Montgomery Ward. According to one report, they did so at first to use up old parts when they changed over to the transfer bar system they called the 'hammer the hammer' safety device.
Revolver Co.' on their less expensive line of revolvers. (Serial numbers were not required by Federal law until 1968.) IIRC, those guns were not serial numbered, so the number may be an assembly or lot number.