I am with Cat Man on this. I recently learned from a fellow member that the case hardening was done in a cyanide bath. I thought they used the old bone and leather type of casing. The base metal in the receiver is a very low allow steel with a not very deep case and a very soft core. Around the filled holes you will get a blended alloy that would harden and temper very differently from the base material.
I would be flinching badly with such a gun wondering if the next time I pull the trigger something bad was going to happen.