spitfire header gasket question

I'm looking to replace the original thick as hell white paper(asbestos? fiberglass?) gaskets that shipped with the old Harold Johnson headers. Anybody know? I wish I'd read the name printed on them before throwing them out. They didn't leak and survived multiple teardowns over the years. They were the small rectangular port 273/318 style.

This is to fit spitfires onto speedmaster heads.

Any info appreciated!!