What he's calling plastic I'd call a paper based phenolic insulator.
Like seen here https://www.atlasfibre.com/sheets/

The cotton based phenolic last near forever. On my truck I'd like to add some terminals to those strips for turn signals too, which will require machining new strips and reusing the old hardware. You could do the same to duplicate the original if you didn't want to buy the reproductions (most that I've seen aren't great quality).

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-