Normally it's just called a cam. You can see it in the picture below, highlighted with the red arrow. It's the crescent-shaped piece at the top, that's used to compress the rubber impeller vanes. It's held in place by one screw coming in from the top.
The back plate, sitting next to the pump in this picture, can also be a wear item, as it can get grooves in it from many hours of use.
Somewhere I have a picture of a worn cam next to a new cam, but I can't seem to find that right now.
Sherwood_pump_cam.jpg
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