It is as Mini13 has said ^ .
Originally, it would have had a brass plug in that hole. I believe the late cars used a rivet but they are prone to leakage.
I drill & tap them 5/16" UNF and fit a grub screw with loctite for sealant.
If you do fit a Brass Plug again, I'd suggest Loctite on that too, again, for sealant, but be sure the hole is really clean of Oil first. I use paint thinners for cleaning in this application.
A word of caution here - What ever you do end up fitting, don't fit anything that's longer than 3/8" or you will blow up your Oil Filter and / or Oil Cooler (if you have one) Just back from the entry point (and not too far back) is another vertical drilling that allows the area behind Oil Pressure Relief Valve to breath. If you block that off, it locks the Valve in a closed position and then the Oil Pressure is only controlled by engine RPMs. I have seen a few exploded Oil Filters from exactly this.
Just one further item to watch. You may get an Oil Leak in the gasket joint between the Block and Gearbox just down from where this hole is. There's another plug here (normally), it's in the sump rail. If the upper plug has been removed (you initial problem), then it's likely the lower plug has also been removed too and possibly missed on assembly. If it has been missed, it may seal off just with the Gasket. There's little Oil Pressure here, so it won't force it's way out, but it may weep.
If you do find this, the engine needs to be split off the gearbox to resolve it.
