DaywhitemtnsI understand that the model is a scaled-up version of the Uintah narrow gauge machine and thus no prototype exists for the mantua model.
IIRC that idea - that it's a scaled up narrow-gauge engine - came from a review in MR (or RMC?) when Mantua first brought out the engine 30+ years ago, but later was disputed, as apparently the model was based on (or at least "inspired by") a standard gauge engine. I can't recall for which railroad right now.
The October 1991 MR had a good cover story on making internal and external improvements to the 2-6-6-2 model.