It is called a "crash barrier", or barrier deflector. It extended with the hook as I recall. Pilots landed with the canopy open (ejection seat envelope not good enough for low altitude ejection back then), it was supposed to keep a barrier wire from entering the cockpit if the plane went into a barrier.
Those of you with the Vertigo Panther will recognize it has exactly the same feature - the "stick" that rises up in front of the canopy when you let down the tail hook.