100V is a very odd ball voltage alright. I wouldn't at all be supprised if it was choosen specifically to keep US imports out. It's much more fundemental than just using a different connector.
Though American 120V light bulbs would last almost forever on Japanese 100V powerlines. But would look rather brown and be very inefficient.
Other American products of old, like the 5 tube vacuum tube AM radio will work just fine on 100V, if you swap out the 50C5 with a 30A5 or 35C5 audio output tube.
On the other hand, air conditioners will be very unhappy on 100V.
