Page from "The Light Car and Cyclecar",1930.

This data was used to determine the design of the lens used in Rotax triangular rear lamp, and in the absence of an original.
The method of manufacture of the lens by Rotax remains a mystery, info please to the author
Was it stencilled and did the light shine through the yellow triangle?  I doubt that.
Here is the the finished job  Rotax Rear Lamp (patented)