What is this hole in the timing cover for?

[Catchb52]

266 v8 has a hole just above the fuel pump that goes into the timing cover. It did not have a plug when I got it.

What went/goes in this hole? Can I just put an expanding plug in it?

 

[FDChappie]

That was for the old style oil fill. When the PCV valve was added, the oil fill moved to a sealed cap on the valve cover.