Here's your photo Tim. No idea what it's for either.
Yaw vane, static pressure port, dynamic pressure probe, 'Odd Rod' IFF aerial, second static pressure port, TAT (total air temperature) probe, and ventral doppler antenna (grey panel)
Whether this patent hints at the reason I know not.
Aircraft control system - Safe Flight Instrument Corporation
Side mounting of the angle of attack vane may lead to a problem when the aircraft is flown in yawed flight. As used herein, the term "yaw" refers to the condition of an aircraft when the longitudinal axis of the aircraft is angled to the left or right of the direction of flight. Thus, the yaw angle or side slip angle refers to the angular difference between the aircraft's heading and the aircraft's direction offlight. Under such circumstances, i.e., when an aircraft is flown in a yawed condition, the position of the angle of attack sensing vane will be determined by the local air flow passing the vane. The angle of attack vanes are usually located so that the changes in the local air flow at the vane cause the vane to pivot in accordance with the angle of attack. The local air flow at these fuselage locations are determined to a lesser extent by the yaw of the aircraft so that the indicated angle ofattack is actually a function of both the angle of attack and the yaw.
In order to overcome the problem associated with yaw, a yaw vane has been used to supply a correction factor. A yaw vane is a movable vane, similar to an angle of attack vane, but which is typically mounted on the outside of the fuselage along the aircraft's centerline. The vane is mounted for rotation about the aircraft's vertical axis so as to align with the airflow past the aircraft. Thus, the change in position of the vane when the aircraft enters a yaw condition is proportional to the sideslip angle.