I think the sega one is the latest one. I do know (Well ok, the story I heard goes) when they made the first Atari one, their contract with Atari had stipulations of when it had to be in stores, so rather than design an Atari on a chip, they used an existing nes on a chip, and ported the games. They may have had more time for the sega one.
Not seeing any nes roms from the first flashback is strange though. Never seeing the insides of one, makes me wonder if it is a NOAC or not.