The prototype would have been cast, not made of pressed sections.
Fox, and other "pressed steel" designs, used relatively thin sheet metal to save material. This was of much more importance in the early days of 'Bessemer' steel -- you see it elsewhere in all those lacy Pioneer Bridge Co. designs and the competing New York City elevated 'mass construction' where an enormous amount of engineering and fabrication were used to require the absolute minimum of then-expensive material.
The sideframes of the current example are cast, and obviously designed to be cast. While in theory you could now weld up the outside section out of plate, it would be ridiculously expensive -- and basically impossible in the days before gas blanketing.
What would be interesting to see in prototype detail is the opposite end of the "J" casting where it attaches to the ear at the far outside corner of the frame. This would be a tension member, which is pulled up at the center with what I believe is a driven-in gib key. How the other end 'actually' attaches to the frame is going to be a detail of great interest (at least to 1ightweight proprietary truck design aficionaderds) The J piece serves both as a pedestal tie and an axlebox clamp; it may be an attempt to reduce the portion of the frame 'outside' the axleboxes (where even a typical three-piece sideframe would have a heavy portion to hold the box aligned).
If this follows the Thielsen patent, the outer part of the journal box will have a long vertical bore, and a long stud will pass from the end of the 'J" piece up to an ear on the end of the truck frame, secured by nuts and washers on each end. It would be 'safer' to have a bolt from the top, to prevent disassembly if the upper nut comes off, but if the lower nut does, the J piece falls down and the axlebox would come loose longitudinally.
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