I also wondered how Kevin did the authentication.
A quick Google search produced the following article on Keating on the PSA website:
Catching Up with Kevin Keating
Apparently Keating who runs
Quality Autographs, graduated from the US Military Academy at West Point with a 4.0 GPA (with a major in Chinese and Far East Studies) and later earned a master's degree from Johns Hopkins.
Unfortunately, Google didn't reveal any other specimens of Joss' handwriting...