I almost forgot - More About Core Plugs

I almost forgot - this is a bit of plot thickening!

I went to the BL/BMC car rally in Peterborough on the 1st August and found a really tidy little '72 MGBGT. The owner was a friendly sort and didn't mind me poking around in the engine bay and taking some photos. By far the biggest news is that his car has a hole at the back of the engine, same as mine but obviously his isn't leaking water, so I'm not sure if there should be a coreplug there or not. If not (as seems the case) then where can the water leak have been coming from? Head gasket? Or was it in fact the main radiator hose which was split when I removed it...?

Not sure but the engine's coming out now so it can be painted along with the engine bay and I'll take some advice when I can get at it properly...