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You'd end up with a meaningful gap between the frame and slide - the Gen4 frame is larger to accommodate the dual recoil spring. Even though you can get the parts together as seen below, Glock recommends against trying to interchange Gen4 frame, slide, mag catch, slide lock spring, recoil spring assemblies, trigger mechanism housing and trigger mechanism housing pin with Gen 1-3 (either direction). Any reason you don't want a G20 with threaded barrel?
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