If you're talking about the one between the top two bell housing bolts, it's definitely installed from the outside. How would one install a core plug from the inside? If you look closer, you should see a step in the bore, unless it's been corroded away. I don't recall exactly, but I think that it's a cup style plug like the ones on the side of the block. Those don't need a step, but are just driven in so the edges of the cup are flush with the block surface.
