Monogrammed Space Chase (2 copies), Ultravision Karate (3), Gauntlet (6), Eli's Ladder (15), Video Life (20), Magicard (23), and Pepsi Invaders (28) all fit the bill, too. Numbers are subject to change. I didn't think Music Machine was that rare, though. There have also been a number of games, e.g. Cat Trax, where a single working prototype has been found.