I haven't got my new brushroll I bought the other day yet so I don't know, I have watched it when I click it forward and back, the belt shifter moves completely to the left and completely to the right, so presumably it does work.
Rogers was only partly shifting to the brushroll driving position from the idler wheel. I think it has 2 belts, one flat belt from the motor to the idler pulley and a v belt from the idler pulley to the brushroll. He said it was the flat belt that wasn't moving across properly. It certainly was a strange design using that narrow V belt to drive the brushroll. Something similar was used in the Dyson clutch assembly, but that worked better.
The only problems with the belt shift was when the wire was distorted for some reason. That wire was perfectly formed to do its job and any interruption to it played havoc with it moving the belt across. A new or at least replacement wire from another cleaner would usually solve the issue.
Perhaps you could photo the belt shifter mechanism Alex - It would be interesting to see it in detail - in particular the "wire" that Benny describes, as this could have been Roger's problem