mymokke sent me a PM (as he cannot post a reply here) stating that the greenhouse and rear panoramic window appear to be the same as the ones of the Wartburg 311-3 Coupé. Thanks for your PM, mymokke.
I had the same idea, but looking closely you can see that the windshield of the puzzle car is more curvaceous than the Wartburg one; the puzzle car's upper door window line is curved, the Wartburg's is straight; the small side window on the Wartburg is larger; the rear window of the Wartburg is divided in three parts, the puzzle car's isn't (I just see that Wartburg must have changed this over the years, so it probably is a Wartburg Coupé rear window). The puzzle car surely was inspired by the Wartburg Coupé and probably by the VW Karmann-Ghia for the lower parts of the body, but that's all for similarity I assume.