it is a guard to depress the radio antenna when the turret is moving.
From an answer on Quora: The A-frame is designed to catch the antenna and push it out to prevent it from interfering with the barrel when the turret traverses past. That’s also why the frame covers both sides of the barrel, allowing it to suppress the antenna no matter which way the turret rotates.
It also prevents a potentially hot barrel coming into direct contact with the antenna.