Nothing...use any of the 40's drums and just get the ring spacer to put on the K-H wire wheels, like I mentioned before.
19Fordy.....I have on my '32 a set of new MT cast iron drums, and they are a little different than the original '40 in dimension....that is to say that I needed to grind off the casting identification # & letters, and make a thin 1/8" spacer to properly fit the wire wheels. The outer extremity of the wire wheels that sit on the outer portion of the drum did not quite match. Turdytoo....do not worry about this paragraph...unless you buy new MT drums.