Here's a pic of the back of my block. You can see the hole you're talking about and what's behind it. Plug or not, it's your choice. It just goes into the flywheel/clutch area.
I don't know the original purpose - maybe some sort of sensor that picks up on the teeth of the flywheel? Or a manufacturing construction hole...
Anyways, I think you can just leave it. I've run my engine without doing anything to the hole - no major
oil spills or anything like that.
Hope this helps.