It isn't like a Singer buttonholer Roger, the templates are inserted at the back of the buttonholer, with a special tool. Without the tool, you have to remove the top cover which involves 2 screws. Apart from the tool, there are a couple (?) of templates missing, and these are special, looking like half a template.
The buttonholer is labelled B-5 HA.L.M, with no indication of what sewing machine models it fits, although it looks like a standard low shank. I've taken some pics (below), the missing piece is number 193, and is a sort of hinged slider arrangement to insert and remove the templates.
I threw caution to the wind here, and spent £10 on it.
I got it reduced as the chap selling it didn't know what it was. I think it will work OK, it will just be a pain unscrewing the top to change templates. I'll let you know when I've tried it!
Jessie