Kevin Adams at Sawmill Creek suggested making a thumb knob/positioner for the removed front knob.

This took about 15 minutes on my wood lathe (I don't have a metal lathe). Tell me what you think.

I used an existing brass knob (purchased on eBay, as I recall), hollowed it, cut off the stem (so it can lie as low as possible), and tapped it for a thread matching the Skew Rabbet Plane.

It feels very natural ...

Regards from Perth

Derek