Re the mystery attachment photos uploaded by John Corden, this is a keyway slotting attachment for fitting to the cross slide of a lathe. As in the final photo, you would be looking from the back of the lathe. Both the links should be horizontal in the photo and a long lever connected to them leading upwards in the photo – which would be the front of the lathe. Thus when standing at the front of the lathe, the back link acts as a fulcrum and the front link slides back and forth. The tool bit goes in the other end of the sliding bar and cuts the keyway (or gear teeth) I am not sure whether this is actually the correct way to contact John Corden or not. If John cares to contact me, I can send him a photo of a very similar item in use.
David Haythornthwaite