It looks like I meant something other than the Maximar, as everything I'm finding now would indicate that the Maximar and Recomar (and Voigtlaender cameras, and many more) used the same backs. So apparently 668/2 would be what I wanted.

But in the event, someone very kindly gave me a 6.5x9 GG back that they weren't using, so the original problem is solved. Now I just need a few more film holders...

-NT