This one is on an early (circa 1810) Mathew Gillespie rifle. I have see a couple of other early Gillespie's with this type of box. Note the "ramp" at the back of the box. Probably to make it easier to "wipe" the patch out.
Here are some later Gillespie’s. The one the far left is a later Mathew (circa 1830’s) with an oval grease hole. The next one to the right is a Harvey G. with a oval shaped hole. The next is another Harvey with no grease hole. The far right one is a Philip G. and the cap box may have been added over the grease hole.
Here is one of John G. with an oval hole
And another of his
The oval grease holes seems to be a Gillespie trait, I haven't seen other southern rifles with the grease holes made that way. They are bored on an angle toward the butt. Some of them show evidence of being opened up a little on the back end, maybe done by the owner who had a bigger thumb!
In any case I believe they were done that way to make it easier to "wipe" a patch over the grease.
I have hunted and used a couple of rifles with grease holes full of grease and I have not had a problem with trash. Of course I haven't used them for years so its no sign that they wouldn't get full of trash.
Dennis