Posted By:
barrysloateThere is a signed cabinet of Henry Chadwick that dates to 1874 (to commemorate his 50th birthday). I don't know exactly when he signed it. It's pictured on page 7 of Smithsonian Baseball. However, that does not qualify as a card.
There are also some signed CdV's of Boston players from the 1870's, including Harry Wright. That comes a little closer to a card.