Maybe the guys with the parts books can clear this up .... Considering the HO72 axle was used for so many years ('47.2 through '72, right?), then are the drums the same for all those years or do the part numbers change?
I am scavenging brake parts off an HO72 as we speak that appears to have two different era drums on it -- one is ribbed, the other is smooth, yet they interchange. I'm assuming the smooth one is much newer (after the bean counters told the engineers to figure out a cheaper way to make them ...).
Can someone with the parts books take a look at that to see when/if the parts nos change? Maybe the part number only changes when they went from ribbed to smooth, but otherwise the drum is the same? (Two different part numbers, ribs/no ribs, yet fit/function remain identical?)
I was told by competent authority that everything throughout the run of the HO72 interchanges (i.e., that it should be a simple procedure to retro fit the Bendix brakes from a '68, for example, to a '48 -- from the backing plate out). I have not proved this to myself, though. Nor do I know anything about interchangeability between SRW and DRW axles)