Anyone know what goes here?

This hole was leaking gear oil so badly that when I drained it there was like a bottle cap in the case. I refilled it today and already it started dripping from this hole. I can't tell if it's threaded or not but an 8mm bolt fits it and turns snug but won't continue to thread. I looked on bike bandit but it only lists the drain bolt. If anyone has any clue it would be greatly appreciated.