there is a seal back there. it's where a distributor would be on a gasser. I can't think of what sensor is it there now. it's just a flat round seal. about 1.5 inches in diameter.
That will be the Oil pump drive.
Take the hold down clamp off(1 bolt) and pull the the drive out.
There's a paper gasket under it but i have replaced it with a proper fitting 0 ring.
It's actually an RPM sensor integrated on top of the oil pump drive on a 1993. Is the plastic housing cracked? If so you need to clean it and seal the housing up with silicone. No, you don't even want to know how much they are new.