There has been a lot of discussion on the stepped tubes. They show up a good bit on 26-27's. The stepped tubes I've seen always seemed to be with the later large drum brakes.
The first Rodda book says probably 24-27. ("I have no documentation as to when this one was introduced. I would say some time in 1924..."). As Jack says, it was used concurrently with the tapered tube until the end of production.