For Viewtec™ windows there is one part here: a 38.6 x 15.6mm adjustable sliding carriage with a brass roller. A brass wheel is there for a reason — it is the roller that wears, picks up grit and stops turning, and the sash then drags on the sill and becomes hard to slide while the window itself stays serviceable. The adjustment on the carriage is what lets a dropped sash be brought back level once the new roller is in. The two figures in the title are the identifying dimensions: take the old carriage out of the bottom of the sash and measure the body, 38.6mm long by 15.6mm wide, because carriages of this shape are made in several closely spaced sizes and a millimetre or two decides whether it seats in the channel.