"Yes, I know the shaft is steel, but the arm attached to it is cast iron."
There's only one version of that clutch shaft with the setscrew stop adjustment. I had one of those welded a couple months back and the professional, trained welder that did the work told me that the arm was cast steel, and it welded easily and well for him. His work looked very, very nice. Based on that input, and that of others here, I'll tell you that the arm is cast steel.