Thats a decent question. The notes state they were converted from steam to HEP when they were converted to 600 series in 1986. So the only thing I can assume is they were used as double headed units with an HEP equipped E60CP, or parked for a while.... (EDIT: Found a note in a photo caption in another book that says they were retired, prior to being rebuilt into MAs).
Another thing to consider (you know, cuz there's not enough already...) is which units had ghetto grilles and which ones didn't.
I also don't know if there was any external difference between converted CHs and CPs after they became MAs. I'd need to stare at a few photos for a while longer.
As the MA's lasted thru 03 and 04, any rolling stock is fair game. From 88 til then, phase III-IVb are all fair game, plus the ugly blotch capstone scheme (MTH made a special run for Nicholas Smith), the MHCs are in play, Viewliners, horizons... the works.