Japan uses a version of the NEMA 15amp outlet, but it's not quite the same.
The two pins are of equal size, non polarised and there are slightly different spec.
The one in your photo is clearly polarized, though.
Some years ago I met a manufacturing exec who had what purported to be a Panasonic duplex 5-15R. It hads an oddly shaped mounting bracket, like it was to be screwed onto a surface rather than into a box. I believe it also had a green ground lead rather than a ground screw. Has anyone else ever seen one of these? He was using it to scare factory workers with the threat of Japanese competition.