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.
Polyfix slide on fitting
Supplied as packed pairs with all fittings