Thanks for the additional detail. I'm thinking of the alleged 'Deshler' gun and some other pieces I've seen, which have a very deep box mortice and the round holes drilled into the bottom of the mortice, so my assumption would be that something would be stuffed in there to keep them in place, and those that I've seen with the deep mortice all look gunked up and grungy so the only thing that makes sense to me is that the balls were pre patched with greased patches. This piece here doesn;t look like that, looks very clean and the way the mortice/lid interact, there would obviously be no need for added material to keep the balls in place.
What a great project!