The track should note there's 2 different versions of this game – 16K (cart and cassette) and 32K (disk).  The 32K version has different credits, a better title screen, missiles that launch from the silos, ships in space that change size, and a larger Zaxxon robot.  Also, the 32K Zaxxon needs to be shot in the missile to destroy it; the 16K version merely needs to be shot.  Several other differences in gameplay also exist (such as not being able to move left or right).