Recently I saw a hole under my car's gear box body. Dust with greasy oil were mixed and closed that hole gap. When I removed them, I saw that hole. I've sent 2 pictures from under the gear box which specified the hole with a circle and something else.
Two questions I have:
1- What is that hole under the gear box body and what is it for?
2- In one picture you see another specified circle around a screw-thread which isn't fastened by a screw. It seems it is a screw-thread of fastening the gear box body. Is that correct which it hasn't that screw? If it is right, what does that screw-thread do?