As a retired researcher and scientist just a cautionary note about the difference between researching and researching with a defined goal in mind. There are serious pitfalls to the latter if one is not careful to avoid them.
I my guess would be that McCullough purchased the gaskets from a third party, to McCullough might not know the gasket composition unless it was directly specified. You really should be looking for best evidence, that being vintage McCullough gaskets and having them tested for asbestos. Provenance is important, as who can say if the gasket on an old McCullough saw is OEM. I'd be searching for old OEM gaskets in factory packaging as they would be best evidence.