Well, that's interesting. I had remembered for my time in Japan -- admittedly fifty + years ago -- that OJ meant using 0 ga track to represent 42" gauge track, resulting in 'larger than 0' models. I see now it's being used to mean 1/45 models on a 24 mm track ga [ 24 x 45 = 42"ga ]; did it change over the years, or did I just misremember it ? [ My wife would vote for the latter....] I could see the 1/45 if one were doing a lot of Japanese standard ga models, but he seems to concentrate on classic JNR. Perhaps his customers display / operate them alongside European prototypes, which are popular in Japan ??
Best regards, SZ