As part of test-fitting the parts for lead-and-rule casting, I found that there is a critical size difference between the American and English pump bodies.
Both these pumps take a ⅞″ diameter piston.
The section of the pump that the Pump Body Lever pulls up on to counter the downward push of the Pump (Piston) Lever is wider on the American pumps than on the English pumps, and the presence of the adjusting screw limits the bulkiness of the ends of the Pump Body Lever. This means that you can’t use an American pump on an English caster (or vice versa) without also changing the Pump Body Lever to match the pump. The Pump Lever might also have to be changed but I have not compared the piston stems to see if they are the same size. The stems definitely look different, with the American one being cast while the English one is machined.
I’ll have to look through my parts stock to see if I have a set of American pump levers. I certainly hope the other end of the levers is the same on both casters!
You can also see in the photo where the English pump bodies are fitted with steel wear surfaces where they slide against other parts.