Another C-4 trans question - vent??

This is kind of a follow-up to my dirty trans fluid question. This trans has a hole about the size of a pencil on the top side of the tailshaft. It definitely looks factory and is not threaded. I guess this is some kind of overflow or vent, but seems odd there is nothing covering it since water and whatever else could splash into it. The area around the hole is covered in grime, prob from trans fluid overflowing out of it.

Was there some sort of vented cap or something there at one time? I don't want to kick some dirt or drop a screw in there....

Most vents are in the top of the case, but that does not mean what you are seeing is not a vent. If you are pulling the tail shaft housing, just tap it out and put a 90* fitting and hang a rubbber hose on it.

OK makes sense. I'm not planning on tearing it apart, I'll probably plug it with something that still allows it to vent while keeping most of the dirt & water out. Not sure what that would be just yet....