Never saw one. The manual throttle "T" handle stands alone in all the military manual illustrations of the dash. You should post a photo of the one you have seen.
There's how you link the actual photo to your post!
That looks like a bubba customized bracket. I can see where it would prevent inadvertent advance of the hand throttle and could probably even be used to maintain a slight opened hand throttle to maintain a fast idle. If your hand throttle is not worn (inner cable to outer housing) you should have no need for it because the "T" handle will stay where you put it on its own. However if it is greatly worn then it will tend to creep out and in on it's own.