A metal wheel surrounded by a faceplate which allows the sash cords to feed through to the lead weights inside the box frame.
Originally the earliest sash windows to date used various pegs to adjust the height of the sash and to keep them in place. Its thought that the system of using pulley wheels and cords was introduced during the 17th century, which of course was far more effective.
Do to its aesthetic value and history, we’ve seen a huge increase in demand for sash windows and restorations. They were heavily replaced by hinged windows which began just before WW1 and through out, until recently.