I've refinished a Manhattan 22, and in polishing down the frame I found three plugs pf copper, one round, 2 rectangular. Knowing that copper melts at a much lower temp that steel, it is safe to presume these were added after the casting. These plugs were not to fill modifications in the original design. Was this type of repair on casting flaws typical practice on small caliber guns that were to be plated? In the pic below, the brass is an access port to install/repair the internals.