Woody, from limited experience with these underhood pads on cars of the 50s and 60s, I would say the material is not asbestos for several reasons.
The factory-made and aftermarket pads I have seen are made of glass wool, usually of a yellow-orange color. Of course they get matted down and blackened with oil residue after several decades of use. The material is fluffy like cotton and irritates the skin after handling.
Asbestos is a white, slighly grayish, soft material. In many forms is resembles paper pulp which has been allowed to dry. It crumbles easily and would not stay intact if folded sharply. It does not have the fluffiness of glass wool. Asbestos sheeting thin enough to conform to underhood contours would be about 1/16'' to 1/8'' thick if a number had to be picked. It is not that irritating to the skin, but is deadly to the lungs if inhaled.
These underhood pads were not a required item, many cars never did have them. They would not do much to keep heat from entering or leaving the engine compartment. The main purpose would have to be noise reduction. The glass wool material was used for its heat resistance. Jute, tar paper, cotton, and excelsior as used in other areas of the car would not stand up to underhood temperatures.
The under hood temps would not reach levels high enough to require the use of asbestos. Asbestos was used for items like exhaust gaskets and brake linings.
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