This is kind of unexplored Constitutional territory. Are Presidents' signatures only valid if written literally by hand? If LIEden ordered the use of the autopen, e.g. to cover up shaky handwriting, are his autopen "signatures" valid because he ordered it? If he ordered some autopen uses but not other uses, are the unauthorized uses invalid, thus invalidating those actions?
One more 55-gallon drum of worms broached by He Who Should Have Been 25th-Amendmented.
I don't know when autopen came to be used by Presidents, but the technology to send handwritten messages in the writer's handwriting was in use in, e.g., hospitals was in use in the late 1970s. Talos Systems' "Telenote" (or "Tele-Note"), a tiny, now long-defunct, company located in Scottsdale, AZ.