This is the horse tunnel on the way out of Stone. Lock 29, whose name I cannot remember now because I have lost my map (in the water), is up to the left. Oddly enough it still rotates. The wooden roller is rotting away and one will soon be able to remove it. I think the local canal society needs to replace the roller so it cant be removed.
This is another picture of a similar roller but this time mounted horizontally on the Stonework surround, which forms part of the bridge wall, to stop wear when the horses cross over the road. It would pull the boat out of the lock as far as the bridge then the rope would slip the looby post and hopefully the boat would have enough forward momentum to clear the bridge. This lock is at a wide road bridge on the Marple flight. The towpath changes side here.