HYDROSWEEP SWITCHAROO
Jon, in reference to your observation about using the 22 washarm in your 20, they are not interchangeable even if you were able to get the top filter/support assembly. The pumps are only remotely similar and while you would get water to pump thru the arms with the 22 support, you would not have the function of the soil collection chamber because the drain impellers and drain systems are different on the two machines. The 20 has the drain solenoid and no reversing motor and the 21-22 drain impeller which was a disposer plate pumps the ground up soils up into the chamber to store and when the motor reverses, the chamber is flushed clean by the jets under the washarm and the soil is washed down the drain.
There is really no advantage to using one arm over the other as the nozzle placement is the same so the washing action would be the same as well.