Xonox was a a third party producer of Atari 2600 games. They made double ended games for the most part. That is, games that you could stick in your Atari upside down to get a different game altogether.
Xonox Games
Artillery Duel
Chuck Norris Superkicks
Ghost Manor
Motocross Racer
Robin Hood
Sir Lancelot
Spike's Peak
Super Kung Fu
Tomarc the Barbarian
These games were all available by themselves, and in double ended
multicarts with other games on this same list.