As late as 1998, one could get an authentic signature from Ronald Reagan's office. With the proper request, his office would affix an authentically signed bookplate into a book - if you sent the right book. Here is a letter on how to do it from his office:
It has been theorized that they gave President Reagan stacks of bookplates to sign... almost as form of therapy as the illness took hold. As you will see in later examples, someone from his office added the recipient's name in calligraphic form, but the Reagan signature itself was authentic through 1998. Requests after 1998 were answered with an Autopen signature. Perhaps the supply ran out and he was no longer able to sign at this point? Signed book examples will follow tomorrow.