I've got the board now. Thanks again, HaaTa! I'd have never found out without your posting, but after some googling it's definitely also a keyboard from an Olympia typewriter.
I don't know the exact model, but the "Olympia Standard 200" uses the same keyboard with several keys missing:
http://www.quoka.de/gewerbe-business/bu ... d-200.html
The switch is different from what HaaTa posted, but probably compatible in respect to keycaps. These are linear though and feel pretty much like a stiffer MX black.

- Marquardt vintage switch
- IMG_3861 (Large).JPG (61.66 KiB) Viewed 3498 times
Made by Marquardt:

- Marquardt vintage switch
- IMG_3862 (Large).JPG (69.34 KiB) Viewed 3498 times
You thought clickclacks are keycaps? THIS is a keycap:

- Olympia typewriter keycap
- IMG_3867 (Large).JPG (50.98 KiB) Viewed 3498 times
A small slit for a big keycap:

- Olympia typewriter keycap
- IMG_3870 (Large).JPG (53.28 KiB) Viewed 3498 times
So unfortunately, this was probably bound to happen:

- broken Marquardt vintage switch
- IMG_3871 (Large).JPG (73.42 KiB) Viewed 3498 times
Nice legends:

- Olympia typewriter keycap legends
- IMG_3873 (Large).JPG (84.43 KiB) Viewed 3498 times
The board itself has no controller, the connector just exposes the plain keyboard matrix (with diodes) and the caps lock LED. With a teensy I could probably get it to to work with a little effort, but I'm not sure yet if its worth it given the layout and the dull switches. And I'd need a casing, too.