This contradicts much of what's on wikipedia, not that it's gospel (though it sounds right to me). According to them, controlling interest of Atari Games was sold to Namco in '85, which sold out to employees the following year; since Tengen wasn't formed until '87, it couldn't have been a part of Namco (nor was it ever anything but a wholly owned subsidiary of Atari Games). In '93, Time Warner again purchased controlling interest in Atari Games, at which point Tengen was killed and games were released under Time Warner Interactive (three years before Midway bought Atari Games).
It goes along with what I said about Tengen being formed when Atari games wanted to open a home division. I might be off about what hapened to Tengen after they were sold, and it's quite possible that Time Warner again bought them (I never looked it up), I just know that they ended up in Midways hands.
Tengen and Atari Games were both operated by Namco for all intents and purposes at the time and were for all intents and purposes the same company, separate from actual Atari Corp.
True, but I'd still guess that this power supply is for an Atari, considering their history and their hatred of licensed goodies.
It's just as likely to be for a NES, Tengen didn't even make 7800 or Lynx games that I'm aware of (I could be wrong).
Namco bought out Atari's Japanese arm in 1974. Before then, they weren't in the coin-op business.
That I did not know, I'm guessing they strictly bought out the coin op division? I know that under Time Warner control Atari launched the 2800 in Japan in 1982, so it's not like Namco had sole rights of Atari products in Japan.
Added in edit:
I re-read what you said about Atair Games and Namco splitting off a year later, I guess that's also correct but the company was basically being indirectly controlled by Namco for quite some time (through the late 80s) from my understanding of the situation.