3 inch vs 3-3/4 inch diameter.
Why is there a difference?
Narrow vs wide nose pan.
Does that mean the 3 inch will fit on either one?
Actually it has to do with the diameter of the fan pulley. Earlier (pre 1919?) fans had a much smaller pulley diameter and therefor use the smaller crank pulley. Later fans with the aluminum pulley are larger diameter and use the larger crank pulley.
Using a large crank pulley with an early fan will spin the fan much faster than necessary.