I thought that a few pictures of these non-cam buttonholers might be fun and useful. I dug out a few representatives from my collection (hoard).
The BHer in the last pix is for an industrial class 31(high shank). It should fit the 31-15 and 31-20 machines.
They are beasts, but work very well and don't need to be removed from the machine to change a cam size. The size adjustments are the wing nuts on the side.
Cathy
[ATTACH=CONFIG]371511[/ATTACH][ATTACH=CONFIG]371512[/ATTACH][ATTACH=CONFIG]371513[/ATTACH]