David,

that's a good one. I was wondering the same thing at first and then I realized that it probably is a "servo stabilizer". The rudder servo wants to be very firm in place and one way to "cement" it is to lock the output shaft in place by using a firm short pushrod anchored to the plate. The ball links allow the servo arm to rotate but the lock rod keeps it from moving fore-aft or side-to-side.

Jamie, please correct me if I'm wrong.

David