Forgive the crappy pic. The 2600 seems to have gone through a lot of revisions before the final product came out. Pretty early proto with built in controllers ala' the Atari 2000. Most game systems of the time were Pong systems that had the controllers built in, so it's no surprise that this control scheme was considered. The controllers appear to be knobs, which would have limited the type of games playable to pong or driving games. Deep sixed in favor of the Atari 2600 with detachable joysticks.