Many vintage British cars use Lucas lighting with push on bakelite knobs.
These knobs slide on a hexagonal shaft with a spring loaded locking button.
This is a collection of knobs with a different faces, plain or one of the letters L (light) W (wiper) P (panel) or F(fog).

The whole project was created in OpenScad.

There are 4 SCAD files: LucasParameters.scad. All of the measurements needed to generate a knob. The only one you might change is "letter" which is the single letter engraved on top. testSize.scad just generates the stem part of the knob, without the mushroom shaped top. A useful and quick way to check that it will slide onto the hexagonal shaft and lock in place. LucasKnob.scad generates the entire knob. If changing the "letter" parameter you will probably need to fine tune the x,y parameters of the 'translate' command in line 47 in order to locate the letter engraving in the center of the knob. myfunctions.scadHelper functions.