I thought it was interesting (as in funny) when reading the PDF on the movietube site that they refer to the reason that the image in NOT upside is because they use a static GG rather than a spinning disk. That aside, how do they flip it? Are they using mirrors like Quyen? Prism?

I wonder if it's a prism with one of the sides ground.

A search here will likely get you to quite a lot of early days dvinfo discussion on the Movietube. My hazy recollection is that it may use a Schmidt prism arrangement, not mirrors. This determines that the device has that signature angle in its optical layout.

If the search under Movietube doesn't get you there, try these two keywords

"bienenwachs" and "schmidt prism"