That is indeed a strake, and not what I had assumed you meant in the OP.
It is definitely for directional stability at high (ish) AoA. It is not on the Rudder, it is on the ventral aft fuselage.
imo.
You don't for silly sake mean the Rudder Trim Tab, do you?
Actuator, I ran across a Rudder AD that originated when a rudder was damaged by blast from another a/c on the ground. Corrosion was found, and a weakening of the riveted join to the torque tube. The area you point to may be an structural accomodation for the upgrade to the rib/tube join. A stiffener, say?
To be precise, the photo here shows damage at the Stab, but led to discovery of damage at the trailing edge of the rudder that warranted a complete rebuild of the damage area you see here, plus a stiffening of the entire lower rib area...to include an enhanced TE flange?
https://www.duncanaviation.aero/imag...ind-damage.jpg