Interesting bit of history. I decided to do a (very) little digging myself into Moses Johnson Robinette. Imagine my surprise when I discovered a photograph posted on "Find a Grave" appearing to be a partial image of a signature from a service discharge, whereupon it is written:

This man is pardoned,
and hereby ordered to
be discharged from the
service.

April 14, 1865 A. Lincoln

https://www.findagrave.com/memorial/33615809/moses-johnson-robinette#view-photo=289786194

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Although I certainly cannot verify its authenticity, if the image posted is that of an actual signature from the discharge of Joe Biden's great-great-grandfather by Abraham Lincoln, then it was signed by the president on the day he also signed legislation that established the Secret Service, which was hours before he was shot at Ford's Theatre by John Wilkes Boothe and died following morning.