(hex:B1) is a glitch item in Pokémon Yellow.Its name is taken from "JACK" in the list of available pre-set names for the player character after selecting the "New Game" option.
This glitch item is also where the effect known as "Jack" got its name in the speedrunning community.
Attempting to use this glitch item may bring up Professor Oak's message that it isn't the time to use the item.