These ROTO u PVC & Modern Timber frame window lock gearboxes come in either a 20mm or 22mm backset, the 20mm will have a 20 stamped on the side. If it is 22mm there will be no stamp, which is quite confusing but it can lead you to finding out which model you have. The fixing centres for the Roto UPVC Window Lock Gear Box is 42mm & also comes wit..