(Mein Deutsch ist rusty, but) If it's this camera, it looks as though it's f/207. In my experience, precision and pinhole exposure is a bit of a laugh anyway. Especially if you're using a negative film, err on the side of overexposure, maybe bracket a little, and you're bound to get something. Once you get beyond exposures of five or ten seconds, reciprocity failure becomes quite noticeable on most films, so doubling the length of time does not really double the exposure anyway.

My 2,

DaveT