In this photo, I can agree with @Phil Coffins that the inlay is to decorate the place where the pin holding the barrel in the stock is located. There is a pin holding the thimble in place barely visible above the thimble. The pin in the inlay is located just about where the pin holding the barrel in the stock would be located. Phil's suggestion about using a piece of paper and a dirty thumb to mark the outline of the inlay. I have used it to make an inlay for my Kennedy SMR replica. You can easily make it from a sheet of brass available from a hobby shop. The inlay itself is likely to be glued in place as I see no pins in the inlay or marks in the recess where the lost inlay was located. The pin for the barrel lug needs a loose fit through the inlay.View attachment 277126
How’s that?