Randy, I have no clue how much a Hubbell plug with that particular config costs, but in one electronics parts & controls catalog I saw a Hubbell Nema 1-15 valise plug go for like either US$10 or $15
Was the cap in your picture moulded-on or was it a "replacement" screw-on plug?
That hole looks like a screw to hole both sides of the plug together, although now I see that it's the stump of the round grounding pin.
Hubbell also owns Bryant. I like their spec-grade duplex receptacles. A nearby Home Desperate used to sell them but then they stopped and switched over to Leave-it-on products