Four parts here are listed as suiting Frontline™ joinery: two 24mm sliding door cassettes, one wide body with an acetal roller and one narrow body with a nylon roller, a keyed window vent lock for current Frontline frames, and a non-keyed window catch and spring. That split is the whole story of the system. The plastic cassette under the sliding door wears or cracks, and the window catch spring loses its bite, while the frames themselves outlast both. Both cassettes run a 24mm roller, so diameter alone will not separate them. Take the old one out and measure the body width, then check whether it sits in a wide or narrow channel. For the locks, confirm whether the existing catch is keyed or non-keyed before ordering, because the two will not swap.