Remember that Blackstone's top loading automatic design lived on in Australia as Hoovers until the 1980s.
500, 600, 700, 800, and I think 900 series.
500 series were basically Blackstone's design.
600 series had a changed dashboard and had an odd neutral drain brake operated by water pressure - when the tub was full of water, a diaphragm down near the base of the transmission was pressed by the weight of water to operate a transmission brake, so the spin clutch slipped and tub didn't drive. As water was pumped away, the diaphragm moved back in, releasing the brake and spin started. The device looked like a large pressure switch.
700 series dropped the water operated brake and changed to an electric pump, neutral drain for one increment, then spin. Also plastic outer tub, very reliable with no rust issues.
800 series - not sure of the change, but I think the cheapening started. ?? Maybe plastic lid?
900 series - change from long to short agitator shaft, so top seal now below water level (bad.)
The first number of the model is the series, the second digit is the poshness level, I think the third digit is mostly 0, maybe 5 if it has a heater??
Others are welcome to chime in with more info about the series and model differences, I may have my facts a bit wrong. (Leon? Nathan?)