What was it used?
I don't think anybody knows exactly when the stepped tube was introduced. Sometime in the final few years. It was used concurrently with the tapered tube until the end of production.
My understanding is, that a few people have looked, but not found any actual record of when they were or were not used. Most of them found on unmolested original cars have been '26/'27 cars. However, enough have been found on black era Ts to believe they were used around '23 through '25 on at least some cars. They have been found on apparently unmolested cars as early as 1921 manufacture, but there is no way to know for certain that a given piece like the torque tube on a given car could not have been changed at some point in a car's past.
The early brass era Ts were mostly outsourced. Ford then assembled the various parts to produce a car. By the '20s, Ford was able to make most of the parts, for most of the cars. However, there always were pieces outsourced, even many pieces that Ford themselves built some or even most of.
It is purely speculation at this point. And I wasn't the first to say it. But the speculation is that the stepped torque tube MAY have been an outsourced piece. It has also been suggested, that they could have been an after-market replacement (Western Auto, Sears, etc). However, that really does not seem likely. Too many nice original unmolested cars have had them, and they are not a part that needed to be replaced often. I suspect they were a Ford accepted outsourced part. Ford probably bought them at about 10,000 at a time.
I had an original unrestored '26 once that had a stepped torque tube.
...and I have what I believe to be an unmolested '27 that doesn't.
I have seen 3 steps and 4 steps.