Kinda wondered the same thing myself. Recently started taking front fenders off my truck and I suspect the foam stuff is what I've circled in the pics. ??
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Matthew 6:33

1952 Chevy 1/2-ton 3100
Late '55 235/SM420/torque tube 3.55
Dalton Highway survivor (using original 216)
www.truckwithaheart.com