"Flat square" historically has been the industry name for conventional "shadow mask" CRT's, ones with a slight curve in two directions like a giant ball. This is the type of CRT used the ViewSonic G810 that the original poster had.
AG tubes historically used "Tron" in their name taking after the Sony Trinitron trademark and are curved in one direction like a cylinder. This is why I could not figure out why everyone was pushing AG style monitors, when the original poster clearly said he wanted a flat square (Shadow Mask) tube.
If the original poster is happy with AG great, I was just tying to stay on topic.
Jim