Reading all the messages lately about transparent back machines... I seem to recall that back in the day (late 70's or early 80's) some PPC member was selling transparent backs for the various HPs.
This isn't too difficult to do - just take a sample back to a reputable plastics shop and they can make anything you want - even glow in the dark. Fabrication today is much easier (cheaper) than it was back then.
I suppost an *original* HP demo unit with a transparent back would be valuable for historical reasons - but it's authenticy could be difficult to prove as it's so easy to make one today.
-J