DieHardCubbie has it!
In order to avoid disputes or confusion when more than one fan claims to have caught a home run ball marking a significant milestone (as has happened numerous times), the umpire will give the pitcher (invisibly) marked balls when pitching to the hitter in question. For obvious reasons, they were named Hank Aaron balls.
Congrats on putting all the clues together, DHC! You really did have to see this question - and these balls - in a different light... such as ultraviolet.