For sale here is a Dark Hendrickson dry fly tied by Roy Steenrod. It is a well-known fact that Roy was the only person to have learned fly tying directly from Theodore Gordon.
This fly comes from the collection of a Sullivan County sportsman who, as a kid, grew up around many of the legendary Catskill fly tiers. His uncle was also close friends with many of them. His Steenrod flies were acquired from a gentleman who received them from Roy's daughter-in-law, Charlotte Steenrod, and I have a notarized letter with Charlotte's signature noting all of the flies were tied by Roy. Further provenance details will be provided upon request.
Dave Catizone has inspected each fly in his collection, written up authentication letters for the framed flies (some for sale here), and told me he believes the unframed, individual flies to be authentic as well.