The '50-'53 Ford/Merc pump pulleys are different diameters to allow the pumps to run at the same rpm from the stepped crank pulleys. The casting difference may be a way to easily ID them.
I think the shape also lines up the belts to the lower pulley, fan, generator.
2 belts on that model. One side does the fan, the other does the generator.
Thanks for the information on the water pumps. The engine that I have in my 53 also has the option of power steering with the 3 belts. Now I am looking for a fan assy. with the narrow belt, any help?