The later pulley I believe is steel although I've never seen one.
Hi,
Both sizes of drive pulleys for the Cub 22 Mowers are cast, they look the same, except one is 4", the other one is 4 1/2".
Yes, 1955 was when they changed to the 4" pulley. I have the mower with the 4" pulley, the pulley has worked well for decades, no problems with it. I had another Cub and mower with the 4 1/2" pulley, it worked good too.
The pulleys have to be kept tight on the PTO shaft, or the splines will wear. The pulleys are made to tighten on the PTO shaft.
There may have been early pulleys that were not made to tighten on the shaft, maybe that is what you are thinking of, V W.
Below are pictures from TM Tractor Parts of the pulleys.
https://www.tmtractor.com/tm-tractor/gpt/drivepulley2_001.htm
https://www.tmtractor.com/tm-tractor/gpt/drivepulley1_001.htm
Below is their listing for a new 4 1/2" pulley.
https://www.tmtractor.com/new/sm/589fp.htm
The smaller hole in the side of the pulley is for a cotter key, it goes through the pulley, and hole in the PTO shaft.
The larger hole in the side of the pulley is for a 3/8" bolt, lock washer, and nut. After putting the cotter key in, you tighten the bolt. The pulley squeezes in enough to tighten on the shaft, if the splines are in good condition. Remove all dirt from the splines before putting the pulley on.