The answer is: whatever was available at the time. Atari used lots of different monitors over the years. Whatever was available when that particular game was put together is what it got. I've seen them with G07's and 4600's, and I wouldn't be surprised if there are others.
All of the upright ones we had came with Electrohome G07's in them. Certainly there could have been other makes/models used since this was at the height of Atari's production. The factories were so busy in those days.