I have always thought that Samhongsa (which also built many 2-rail O and I believe HO
brass models) built most of the Williams brass offerings, except the very early pieces
(the too-maligned early Dreyfuss was one of them) and the late Masterpiece units.
Samhongsa had quit the model loco business before the Masterpiece items were built,
as I recall.
But I don't know that for sure, and Samhomgsa certainly did build a lot of Weaver items. Anyway, interesting. I wish someone would do a little definitive book on 3-rail brass loco/rolling stock production. It has been going on long enough to have a plot.
When I stated way above that "there's nothing special about these things...", I actually
mean ALL model locomotives, of any brand. You strip all the electronics - simple or
complicated - out of a Lionel or MTH or 3rd Rail loco and you are left with a motor
and some gears, and a place to be filled as you wish.