As a quick follow up, while sitting here after writing the above, I pulled the cover off the encoder assembly on the servo motor and with it plugged into RA there is considerable end play in the shaft and gently fiddling with it the motor will rotate for perhaps a bit less than 1/4 turn and then stop. It will jump around a bit gently fiddling with the gear. End play would be something north of a millimeter I would guess. Not enough that it contacts the read head but the end play will take up pretty much the entire gap in that head.
