I recall from my childhood, a gunsmith up on the river near Hachioji between Tachikawa AFB and Yokota AFB. He sporterized a couple of rifles for my father. One a Springfield 03-A1 and the other a P14 Enfield. Not the best work by any means, but it was probably pretty classy for my MSGT dad in 1967. What I recall about the shop was that all the machines were run by flat belts that led to where the water wheel used to drive them. The water wheel drive had been replaced with an electric motor, but all the machines were ancient at the time and the floors were oiled dirt. A gunsmith was heating something in a charcoal furnace outside and forging it.
That was the only time I saw any Japanese gun related things while there from '65 to '68.
