Anti-balance tabs & pilot feel.
I am not sure whether the B707 ever did not have hydraulic boost to the rudder system or not. What I do know is that the rudder is the only flight control surface on the B707 which is hydraulically assisted. It normally has 3000 psi available at low speed, this being reduced to 2250 psi at speeds above 250 kts. The pilot feel, when hydraulic assistance is available, is provided by a "Q" feel unit and spring at speeds above 130 Kts. The purpose of the anti-balance tab is to make the rudder more effective. For what it is worth, rudder displacement from neutral available when no hydraulic assistance is available is about half that available with hydraulic assistance. (about 13 degrees without, 26 degrees with).