History 1960's Sports & GT Symbols used in European Racing

Seeking more information about decals/stickers used in European Sports & GT racing in the 50's & 60's. They were used to denote engine size. As I understand it, it allowed other cars to know if the car ahead was in the same engine class. I believe there were a few different 'shapes'. A cutout star, a cutout square, a cutout diamond and perhaps another shape. The cutout allows the body color of the car to show through. The background I believe was either white or yellow depending whether it was used on Sports or GT classes. I've searched and can not find more information on line. Thanks in advance for any insight.

 

What race was this? 1965 Rheims 12-Hour? Schlesser Daytona Cobra wore #26 in that race but I don't remember a #27 Cobra. Backup car maybe? Car #27 that year was a Ferrari 250 GTO driven by Sears/Whitmore.

(Carroll Shelby and Jim Hall both ran Texas license plates on their respective cars.)

 

What does this have to do with traditional hot rods?

 

Yes, I believe this is '65 Reims. CSX2299 driben by Sears/Whitmore. 9th OA and 2nd GT. https://www.racingsportscars.com/results/Reims-1965-07-04-2425.html

 

There are quite a few FB sites that you might search for, historical sports car racing-wise.