Technical Leaky GMC six

A good friend of mine just picked up a beautiful 49 GMC pickup truck. The other day it started to drip antifreeze. Upon investigation it's coming from what looks like a rivet head about .75 inch diameter located just below the core plug on the driver side. Does anyone have an idea what this plug is?
Thanks

 

I believe I would clean that area up real good and J-B weld it .Let it sit 24 hours and proceed.In this instance I reckon it cant hurt anything and might well stop the leak.I assume you are not getting any coolant in the oil?

 

I don't remember a rivet there. Or know why it would be there. maybe Jim Tone knows something. But maybe some Bars Leak would work.

 

Rich, I looked at a few blocks, some have them some don't. I'd clean up the outside real well and put in one of the GM tan pills about the size of a quarter and run it until nice and hot.
As for a repair, drill it out and tape a pipe plug and then us it for a drain.

 

Could that be a rubber plug that got hard? I recall rubber plugs with a hole in the middle that could be tightened with an Allen wrench that had that type of head. I've found Zecol Mendtite seals antifreeze leaks in pressurized systems without plugging heater cores or radiators.

 

That's no rivet, that's part of the casting. Any fastener used to hold the casting pattern together shows up on the actual casting. I'd be taking a closer look at the freeze plug right above it.