[pat in Santa Cruz]

the one with the holes is a 31 cast iron drum. If its model A it is 11 inches. '32 and up were 12 inches. The guys are correct that cast and steel have differing friction coefficients and both drums (actually all 4) should be the same. Cast rear drums in 31did not have the holes, but they do not have the curved rim that a steel drum has. Bert's might have an original cast drum you can match up with. Cast is the better drum as it offers more friction as well as less expansion. in the long run, you would be better off with new cast iron drums all around and soft bonded lining. my31 slant has original cast drums on front, and Randy's on the rear. It stops on a dime