Thanks for your attempt, Leonhard, I did try to describe it in the comment! Unfortunately, I do not have a link that I can post here, and it does not seem to be possible to attach an image to this thread...

re #2 : könntest Du dieses Bild hier verlinken ?

Ggf. vorher einscannen und hochladen, z.B. bei https://addpics.com/ (E-Mail-Adresse zum Hochladen und Erhalt des Bildlinks nötig, aber keine Anmeldung oder Cookie-Gedöns für andere zum Ansehen)

I don't have a clue, but I did find

dumbbell spanner (Br.) / dumbbell wrench

in another source. Would that work?

google for pics

@ RES-can: nope, does not look like that, that looks like a Dogbone wrench, which I already ruled out...

@ blaugast: actually, it does look a bit like a Maulschlüssel!

@ Leonhard : seeing as it dows look like a wrench, your last suggestion might actually work!

I tried the addpics thing, they took my email address but did not send me a link to the picture... :o(

Hmmm ... ich hatte bei denen noch keine Probleme mit den zugesandten Links ...