Machine shop forgot oil plug?

This plug is seriously in there - I'm giving it big wallops with my 5lb sledge and it's not budging or even denting on the main bearing side. That much force without any movement is the point where I say STOP and double-check that there's nothing that I could break by continuing to hammer it - for instance, there's no possibility of anything but the plug being in between the sender hole and the opening on the main bearing cap?

That sounds like the point I would decide there's not a plug in it at all.