FWIW, calling it a “trim tab” implies it would be trimmable. It is fixed and not adjustable. It simply increases the chord of the airfoil and therefore the thrust-- remembering the equation for lift.
I believe it was added as an appendage starting with the BA version primarily as a consequence of increased power/gross weight. The yaw-load compensator followed shortly thereafter on the B1.
Initially the tab was attached via rivets, and subject to a recurring inspection for cracks initiating at the rivet holes. The rivets were eliminated sometime around the introduction of the B3 Mod or B3/2B1 aircraft. To my knowledge, the majority of the newer, rivet-less tail rotors are painted gray while the older style are typically blue.