Virtual Boy GameLink® Cable:

Although it was supposed to be released shortly after the American launch of the Virtual Boy, it never was.
A couple of games like Mario's Tennis and Space Squash were although not visible in the game menu's rumored to be made with a 2-player option in mind, but none of such has ever been proven as of yet.
Serialnumber: ?
Nintendo of America Part No. 32557 (unreleased, no price was ever given either)