After making the shift, directional, and dimmers knobs, I've had several request to do the heater knob. Unfortunately both my Isetta's are missing their heaters, so another Isetta owner graciously loaned me a heater knob to copy.
The picture on the left shows an original heater knob next to my reproduction. I'm not sure what the story is with the color of the old knob. They usually matched the color of the shift, dimmer, and directional knobs. The shape of it is correct. Now that I have heater knobs, all I need are 2 of the under seat heater assemblies. The picture on the right shows the complete set of Isetta knobs.