At first glance I didn't think it's gun related, but I couldn't prove it's not, what bothers me is the size of what would have to be a single screw hole to attach it, and looking at your fingers to size it, it seems to be a little small overall, but again, that might not be valid either, there were all sizes of pistols, from large to very small. It's possible for it to have been a butt plate, I know there were flat butt plates on some muzzleloading pistols, I've seen cased French caplock dueling pistols with flat butt plates, so just because it doesn't look right to me doesn't mean it's not. Is it brass? Some part is broken off, and that doesn't help with the ID either. If it's a butt plate, it wouldn't work with only one screw, there would have to be something to keep the plate from turning, which is possibly what the missing part was for, or perhaps there was a finial that went up the stock and strengthened the grip if the gun was being used as a club, which essentially is what early muzzleloading pistols were after the single shot was fired at close range. I'll be interested in what others say.
