For Vision™ sliding doors and windows, one assembly is listed: a 12mm adjustable plastic carriage with an acetal roller, supplied in a 2 pack and also listed as replacing Stegbar® rollers. Sashes normally run on a pair, which is why it comes as two — replacing only the failed side leaves the panel sitting unevenly. Nothing else is listed under the brand, and that is the message in itself. The roller is the wear part, and once the acetal wheel flat-spots or the adjusting screw seizes, the sash drags on the track instead of gliding. The 12mm in the title is the identifying dimension, measured across the roller, and it is the only thing separating this carriage from the 13mm and larger versions that look much the same. Measure the old wheel before ordering.