The first Thunderbolts built had only a fiberglass skin resembling a bumper. This was apparently ruled by sanctioning bodies as not being legal, so an aluminum bumper was produced. Some cars had the metal bumper installed overtop of the fiberglass piece, and others trimmed or entirely removed the 'glass part.
Here's a photo of the aluminum bumper from the Hi-Risers Thunderbolt. The difference in weight from the aluminum bumper to the same thing in steel is amazing.
More to come...
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