You are correct Max; I was being sloppy. Roughly the /64 correspond to the original FVM wheel width and the /72 to the wide FVM wheels. NWSL also produces /50 wheels, but not for locos, just rolling stock (28", 33", and 36" dia.). At $12.95 per 4 axles, they are a bit pricey though.
I don't think $12.95 is pricey. That's $3.25 per axle, and after all, they are a specialty item. If they were making them by the hundreds, I might expect them to be cheaper. But I bet they are a very low volume product.
