A few notes:
- I believe the Happ opto boards are pinned differently, and in order to exchange them with the Atari ones, you need to swap a couple of the wires on the connector. (I forget the specifics, but if you search, you can find them.) Just FYI, if you're trying to mix and match, as I don't know how the MH roller was designed, and if whatever harness you're plugging it into may already be reversed.
- Note there are two variations of the Atari opto board. There are actually a ton of PCB revs (i.e., the letter in the square box), but they all have the same electrical layout. They can also be hand-labeled -01 and -02, however the main practical difference is the resistor (which is the only other thing on there).
Some of them have 100 ohms, and others 150 ohms. I've read that this is supposed to correspond to the -01 and -02 numbers, but I'm not convinced of that, as I was actually (coincidentally) just looking at this today, and I have some that appear original (no signs of rework) that are both labeled -01, with different resistors.
I know there are also some with 2 mounting holes drilled on each side, while others have 3. Again, I'm not sure if this is what the -0x corresponds to, and I haven't dug into the manuals to try to cross reference the part numbers. I also don't know which game(s) used the 150 ohm resistor vs the 100, or if this makes an actual difference. I do know the Tempest schematics show 100, so there's that. This is an area where IMO more research needs to be done, but I just wanted to give you a heads up, in case it's making a difference in your case.
(Actually, here's a link with some info about all of the above. Bob Roberts had a page too, but I can't find it at the moment: )
http://www.arcaderestoration.com/Articles/1/Bob+Roberts/59/A+Look+at+Trackballs.aspx