Looked at mine this morning while I was messing about with relays. I have a tapped boss (like yours), with nothing in it (like yours), the only difference is that mine isn't leaking oil (unlike yours) !
I would suggest that it needs plugging, but I am confused as to why it would suddenly start leaking.
Is it possible that the oil is being blown up and along from the bolt holding the rocker cover on and nothing to do with the boss ?
I honestly don't know where its coming from! will have to clean it and see where it appears from first.. I think when I bought the vehicle it appeared to be coming from the hole in the cover.
Will be a while til I get around to looking at it, now the damned thing is flattening the battery over a 2-3 day period! Have read the forum about it and have a lot of things to check!