What part of a 3 speed overdrive trans is this?

[LowGT]

Recently bought a 3spd OD trans with all the wiring and levers that came out of an original truck. Things got a little hairy during removal but i didn't see any damage. Now I notice something was broken on the top side of the tail housing, I'm hoping it was just a vent but I'm not sure. I can see the output shaft through the whole. Think I could just weld this hole closed and forget about it? I attached an image of another OD trans with the "thing" there. Anyone know what it is/ if it's important?