All polyplastic window fittings are available in either a 'standard' screw-on fitting or the more recent variant of the slide-on 'Polyfix' design. The slide-on 'Polyfix' design are easily recognisable by the deep grove moulded in to the back of the window fitting. It is this that slides over the adaptor fitted to the window. Screw-on fitting Supplied as packed pairs with fittings