I guess I am more in need of an Autopen expert than a signature expert.
Since you were willing to do some research earlier, I again recommend checking out through interlibrary loan the presidential autopen autographs book linked to in an earlier post. It's cheaper than $100. Keep a photocopy of the Eisenhower section with the letter as your proof one way or the other.
Whatever you do, I think we'd all like an eventual report back on your findings.