Changed my oil today in my 82 300TD. Replaced filter and plugged after draining the oil..All business as usual, and I thought the job was done.

Five blocks from my house when driving I noticed I had no oil pressure. I pulled over to check under the car and that the oil drain plug was still in place. Plug was in but between the transmission and the oil pan the engine was hemorrhaging oil. I think maybe some sort of rubber stopper/plug came out, but cant find out what the part is. I put my finger in the leak and their isnt any threading so I'm assuming its some sort of grommet, but can't find anything online. The hole is about 1/2" wide and I can feel a pin running horizontally to the ground shallow when i place my finger in.

Any help would be appreciated...ug.