|Black Forest bobs
There is no reliable rule to determine what size of bob to use on a Black Forest Clock. It is normally a piece of wood covered in a thin brass facing. The bezel size helps. The 10" normally takes the larger 75mm bob. The 8" normally takes the smaller 55mm bob. Anything in between needs to be considered as 'the nearest' to those. This is only a guide.
Repairing Your Own Clocks by Mervyn Passmore