I can't answer your quesiton, but I do have the same problem in my circa-1950s house. About half the outlets are 2-prong. Most/all are polarized, but some have been painted over so many times that the holes have become smaller.

It turns out that much of Japan uses non-polarized 2-prong outlets very similar to ours. Cisco part number 72-1925-01 is an IEC cord with 2-prong connector and a grounding wire that's about 4" long. It has come in handy on a few occasions and they're not too pricey.