Some time ago I drew up the complete gear lever assembly from original drawings and I can share the knob cad file with anyone that wants it but I am not a cad expert so it may not be 100%; just send me a PM with your email address and I will send you a copy of the file.
Best is to use the...
Couple pics of the knobs I had made by a friend I am not very good at turning wood, one Delrin and one walnut. As Jimmy mentioned Ebonite is still available, we used Delrin because it turns like wood and it doesn't change with sunlight.
I think it was made by Hella in any case here is one and yes it was used on some Aston Martin cars.