Did you find the 55 here in California or are you from back east, too? We just sold our 55s in June to a safe home. See my post in the FOR SALE section under "55 with a spare". Ours were late models, but my dad run them from 1949 until 1994, then went straight to the 9400, skipped all the 00 & 20s. It is quite a shock to just flip the switch and all that HP hitting the system. Every once in a while, like maybe once every two years, I forget to have the engine speed in low position and hit the machine switch, man that hurts.
I should qualify my answer on the 7700. I have run 7700s and 8820s and did not see any reason you could not engage the unloading augar, but never unloaded on the go. There may be something I don't know about.