Booster Grip
From Videogame Interface Wiki
Sleeve that fits over the default Atari 2600 joystick, and adds an extra fire button. Came packed-in with that system's version of Omega Race. The homebrew games Thrust, Thrust+ DC Edition, and Thrust+ Platinum also support the accessory.
See Also
- Booster Grip on AtariAge.
- Omega Race & Booster Grip Atari 2600 Review - The No Swear Gamer Ep 247 - Youtube video showing the game and controller.