First built during WWI, these heavy duty 70 ton trucks were used under a variety of PRR equipment (F25, F25b, FN, FNa flats, G22, G22a gondolas, and H21a, H21b, H25, H22a hoppers). The sideframes were cast steel and included metal spring planks. They were built along Andrews design parameters, but had some special feature(s) that encompassed the Crown patent. Most noteably associated with PRR's H21a hoppers, these were tough trucks - with some lasting all the way into the 1970s.
The H21 hopper dominated all other cars on the Pennsylvania Railroad. The road moved a lot of coal, coke, and iron ore, so hoppers were essential. By June 1951 there were just under 40,000 H21 hoppers (of various subclasses) on PRR's roster. These cars date back as far as 1909, but the majority were built between 1915 and 1918. A significant quantity of them still existed into the early 1970's, but they vanished pretty quickly after that. The H21a subclass was the most abundant at just under 32,000. The H21a is a four-bay hopper that could hold 2,547 cubic feet of cargo. Its 2E-F2 trucks were much larger than the archbar trucks used on the H21 class (necessary because the H21a was used for hauling coal rather than the lighter coke). All H21 cars were converted to H21a by 1925. The H21a represented the PRR's first large scale use of cast steel freight trucks.
Bowser made the first 2E-F2 truck in N scale. I'm told that it's somewhat inaccurate (insofar as it lacks a spring plank) -
Bowser - wheelbase 10.73mm, frame width 16.05mm, wheel size 33", axle length 13.92mm