Pictured above is an NOS Model B oil pump shaft. The Model B had a somewhat more 'pressurized' oiling system than a Model A, as well as bigger diameter main and rod bearings, so the shaft was reduced in diameter to 0.410 inch (about 13/32) to provide for increased oil flow within the pump internally.
Model A oil pump shafts were just straight 1/2 inch diameter shafts with no reduced shaft diameter section.
Pictured above is page 67 from the 1932 Ford Service Bulletins, showing the shaft identification and construction.