That looks like my APE top that I bought way before the MkIVs existed. I did the drop butt mod on my MkII which gave a rear shape similar to a MkIV. I closed that gap w/ foam insulation.
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I made the rear window easily removable so the top was always on the car. Side and rear windows in or out depending on the weather. BTW the rear side windows I added were a huge improvement.
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