If it is a 'Normal' 9x12, another solution is a spring back addition sold in the 50's and 60's to convert this type camera to 3.25x4.25. Has a built in ground glass, spring back and allows you to use standard 3x4 graphics Type V film holders. You can still get 3.25x4.25 film from freestyle. Look on the Bay.
What do you do with a plate camera that small? Cut 6x9 sheet film in half?
I don't know why I'm even asking---I'll be flayed alive if I bring home another camera. It's safer if I just don't know that such things exist.
If I feed it well, maybe it will grow??
Cutting 9x12cm film in fourths is just as easy as cutting 6.5x9cm film in half, since the smaller films are 0.5cm too wide and must be cut down anyway. The good thing is that such a small piece of sheet film is stiff enough that it doesn't need a film adapter - just put it in as if it were a glass plate.
... I don't have my library with me, so I don't know what kind of lens the Ernemann "Doppel-Objektiv Serie A" is, but I know I can find out. ...
Reunited with my library, I find that the Ernemann Doppel-Objektiv is what I should have suspected from the start: A Periskop.
That's a lens consisting of two elements symmetrically placed each side of a central stop. Since these are not achromatic, a small aperture is necessary to give satisfactory performance - the f:11 should have been clue enough for me: Wide angle lenses and Periskops were generally the only lenses to come with a max opening of f:11!
BTW, I'm moving this thread to "Plate Cameras", since that's what it is.