Looks to me like that left barrel blew and the area was pounded down, the hole trued up and the hammered area engraved. Then the hole filled with the brass plate. With the rifle barrel inside the bore, it is not an issue.
I looked through my father's old loading manuals, but none listed the pressure the .33 Winchester was loaded to.
In addition to Colt and Lefever, E. Remington & Sons offered both double rifles and cape guns in their Whitmore lifter action doubles. When they went to the top snap doubles in 1882, they began offering the auxiliary rifle barrel. They continued offering the auxiliary rifle barrel into the Remington Arms Co. years.
Auxiliary Rifle Barrel 1901 Catalogue.jpg
Threaded Mode