Oil pressure relief valve bolt question

Here's a picture of where the oil pressure relief valve goes in, but I have no idea what the bolt is directly above it. Does anyone know?

Sam, I would like to say its the drain plug for the coolant..? I am still getting to know the 1800 engine...

It's an oil hole plug.

When designing oil paths occasionally you need to have a drilled path to connect internal passages. The hole used for the drilling is then plugged. In this particular instance this doesn't appear to be the case. Just bellow the relief valve is an actual pressed plug for this very purpose. If you look at the internal schematic, the upper passage appears to connect to the same passage as the outlet for the oil filter or oil cooler on the other side. In theory you could have a fitting in this hole route to the oil cooler and then back to the oil filter on the other side. You would also need to plug the hole on the other side. Since this engine casting was used for many, many, years it is possible that this routing did occur in an earlier incarnation. Case in point is the casting for the engine crank near the front of the engine on that side. Not much use for that on the MGB, but there was no point in modifying the casting.

Accoriding to my '65 BMC dealers parts book it is a block off "screw" for the oil galley. I have seen in the Moss catalog where it is incorrectly listed as a "coolant" drain plug. RAY

Thanks for the responses(and history lesson) .. I'll pull it out for cleaning.