Quoted from L_satan:
Good luck, Tony. I'll be starting work on Middle Earth soon, and let you know what I learn. A Gen 1 system, but circuitry is probably similar.
Gen1 and gen2 systems are two very different animals when it comes to their boardsets.
Quoted from MarAlb:
I think this counts for the earlier Generation 1 games like Airborne Avenger, Space Riders, ...... The boards in Generation 2 games (Superman and Hercules) are the same. I never saw differences or different populated boards.
To add to that, the only difference between game-specific gen1 MPUs is that they were only populated to the point where only the lamp circuit ICs and solenoid transistors that were necessary for the specific game were actually installed on the board (a cost saving measure, I would guess). Supposedly, if you fully populate a gen1 MPU, you could turn it into a universal board and then just change the ROMs for the game you want to put it in. When I get some time and a spare board that needs repairs anyway, I was thinking of giving this a try since I have two Atari games that I could test this theory with.
According to John at John's Jukes, he has (or had--not sure if he sold out of them) NOS universal gen1 MPUs, so they do exist.
However, as for Gen2, I acquired two power supply boards that had slightly different layouts. I'm not sure if other variations exist for the other boards or not.