I'm pretty sure bare is right.
It looks like the site for a tapered pin expansion plug.
Often it is necessary to drill holes for oil or air to get it from one place to another. The only way to make these transfer holes is to drill them from the outside of the housing but this leaves the hole where the drill bit entered the case.
These small expansion plugs are then inserted into the mouth of the hole and usually expanded with a tapered internal pin.
For a look at these pins follow this link:
http://www.leeimh.com/metal/betaplug-expansion-plugs/betaplug-expansion-plugs.htmAs bare suggested, degreasing the mouth of the hole and epoxying in a metal plug or rivet that fits will fix the problem.