3rd coolant drain plug on block?

Why is there 2 drain holes on the drivers side of the block? The one circled in this pic is smaller than the other one on this side. Do I just plug it? What size is it? It's smaller than the other 2 coolant drain plugs and the 3 oil galley plugs in the end of the block.

I'm assembling a FRPP M-6009-B50 short block.

Rick

Does anyone know anything about this??

Rick

Update:

I checked my block and there was the cast-in boss there, but no hole. I wonder if, since this shrot block assy. was destined for the explorer, it may be a feed hole for something like hydro-boost brakes??

Anyway, I'll be plugging it once I figure out what size it is.

Rick