I just blogulated on Steele Rubber quality.
https://www.stovebolt.com/ubbthread...3332/re-starter-bellows.html#Post1303332

Durometer does not have much to do with a windshield seal. Durometer is more of an interest in clamped gasket seals, flat gaskets, orings and such.
The windshield seal is a overlap flap and groove seal. Almost a static "sweep" seal. Watershed kind of thing. There is no mechanical compression/clamping.
Obviously the seals can't be rock hard or spongy soft. The dimensional stability yielded from the mold will make the product fit better. The quality of the rubber will make it last longer. Those are the main factors regarding off shore vs USA.

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