Look at the floor of the box of RCA 21 (iirc off the top of my head). That hole comes in from under the butt and goes forward quite a few inches, I think it's around 7/16" or 1/2" diameter. Unfortunately, the box cavity ("oops") cuts into the hole.
Now, I could probably go on all day and really rile things up with my own personal thoughts in regard to the interaction between that hole and that box mortise. I'll leave that be...
Nevertheless, that hole is a perfect example of what I'm talking about, has been found on a few of those pre-19th century Berks rifles and some others too. It had to serve some purpose, just don't know what.