Hi, IMO the missing brass may be because of galvanic action over the years between the copper bearing alloy and iron. It would have been nice to have a soil sample adjacent to the revolver to test for copper salts? Even in the condition of the ferrous metal part it should still be able to find a serial number. If the serial number was stamped into the handgun, even if the surface rust has appeared to remove the serial numbers, careful sample preparation would allow etching back the serial numbers so they can be viewed.
Regards,
Richard