What does the asterisk before the serial number in the 5 1963 bill mean?

The asterisk before the serial number on a 1963 $5 bill indicates that it is part of a replacement note, also known as a "star note." These notes are issued to replace misprinted or damaged currency, and the asterisk serves to distinguish them from regular notes. Star notes are often collected by numismatists due to their rarity and uniqueness.