I think that I should pull some of those outlets off & see what I have got going on back there. . . .
I would be very surprised if any of your 3-prong outlets have a 3rd wire ground connected to the outlet’s ground terminal (many 1960’s house outlets don’t). Someone at some point probably replaced many of your house’s 2-prong outlets with 3-prong outlets. For some reason, that 2-prong outlet that you’re referring to was missed.
I take it that the 'standard (non-isolated ground) outlet (which uses the conduit or metal clad cable ground)' you referred to will have the 3rd ground. I wonder if that is what they did in the rest of the house where the outlets do have the 3rd ground?
My guess is yes, that is the case.