Well, as hoped for, the rubber windshield surround is, in fact, press fit into the windshield surround trim channel.
However, the 964 part number (that PET shows for the '89 Speedster, which I assume is a supersession), has a second mounting point, that the windshield frame does not have a provision for. I can only assume that Porsche expects you to carefully "remove" the, now superfluous for this application, outer mount.
Regardless, fun times this morning...
