Boeing found they weren't going to be able to put enough rudder on the airplane for it to be as controllable as they would like. Instead of redesigning the whole tail and increasing the size of the entire rudder them simply cut it in half (vertically) and added the tabs to increase effectiveness without increasing size.
727 Prototype Airplane Historical Photos and Data
You can scroll the the photos of the prototype here. You'll notice that on the first flight the tail and rudder all match in color, but there are photos interspersed of the aircraft at a later point with an all white rudder, which is the rudder used on the production aircraft.