Authentication: Know how people signed their names

Is it possible that there is a letter signed by Lincoln with his full signature, Abraham Lincoln?  He never signed letters anything but A. Lincoln, though he signed legal and presidential documents in full. Could he have slipped up once and signed his full name to a letter?  Maybe, but is it worth taking the chance?  The same is true for Patrick Henry and George Washington.  Never will you see their names signed in full except in the text of a legal document or receipt in their hand, and very rarely then. Know how some people signed their names, and you are less likely to be stuck.