I don't know how long that loco was in production, but Max Grey was around for quite awhile. I think somewhere in the 60s max sold out to Levon Kemolyan (Kemtron) and he renamed the company US Hobbies. That lasted I think into the early 80s and I think Levon died. Westside Models, a competitor bought the contracts/business and kept it alive a few more years at best then folded. So you probably have a 30 year history.
The models such as your William Mason imported where not made continuously. The company would make a "run" of some number and import them and sell them. So at any given time, the product available varied quite a bit.
both MG and USH did import an extensive line of freight cars but no passenger cars that I have seen. the cars came unpainted and without trucks and couplers. Decals and trucks were sold separately. They only did about a half dozen diesel models over the years but a very extensive range of steam loco models. If a model sold well, multiple runs were made.
Due to some tariff laws, the locos came in modules and required a little screwdriver assembly. Generally the boiler superstructure was one unit, the mechanism another, and the weight had to be installed. the tender trucks also had to be installed.
In their day, the MG and USH were considered the top of the line brass imports. the models still stand up well today, but are not as well detailed as more recent imports. However they are very sturdy compared to recent imports.
A final note, I was told that MG and USH had an exclusive contract with KTM to build the models. during that era, KTM built only for MG and then for USH. KTM was considered the best Japanese builder.