Rich,
Your model is excellent, but I am not sure about one aspect of the swing motion. All of the swing motion trucks I have seen have the swing hanger upper pivot bearings mounted on top of the upper truck cross beams. I don’t know if this is to lengthen the radius of the swing or to provide some extra insurance against dropping the spring plank if the upper hanger bearing should happen to break under tension if it were hanging beneath the upper truck cross beam. There may have been some swing motion trucks with the bearings mounted on the bottom of the upper truck cross beams as you show, but I have not seen any. If you have seen it that was on the Knotts truck or elsewhere, then it confirms that they were made that way. But I cannot see that detail in the photos of the Knotts truck.
From what I can see on the Knotts truck, I am inclined to believe that they have been rebuilt to eliminate the swing motion. Notice where you have those steel bars that make a bolt splice between the top of the truck cross beams and the top of the side frames. They are on the Knotts truck, but on that truck, there is also another steel bar that is right next to those splice bars just inward from them. Those other steel bars, which are not on your model, appear to have a bend that allows them to drop downward on the inner sides of the truck cross beams.
My guess would be that these iron bars, with each pair that are directly across from each other, actually are one continuous piece that is formed like rectangular stirrup. And that stirrup is what is carrying the spring plank at each end. The form of those stirrups would be that of an inverted hat form.
If my assumption is accurate, I would assume that those fixed, non-swinging stirrups mean that the truck is not a swing motion design. Maybe it was originally built as a swing motion truck and then later modified to eliminate the moving parts of the swing motion, or maybe it was originally built as it appears today and never was a swing motion truck.
