I played a lot of Mike Tyson's Punch-Out as a kid. I wasn't any good at it, but loved playing it. With as much as I played it, I don't think I ever actually beat the game. Now, this Easter Egg isn't an easy way to beat the whole game, but it is a visual clue to help with certain characters in the game. And as far as I can tell, this has gone unknown for many, many years.