I was just looking in the owners manual for the 8590 that I am going to try this fall, which has the same rotor as the 9720, it also says 31.5 by 168 inches.
You're right! 8590, 9720, and 9700's all have the 31.5 X 168 inch rotor. I know from helping my neighbor pull the rotor twice on his 9700. It's unfortunate they didn't continue to build this machine!
Was at a Massey combine clinic the other day, and they claimed that the large rotor machine was coming out again, posibly in 2002. Said that it would be class 8 and have the hydro drive rotor. We'll have to see if it happens or not.