By seat belt I suppose that you are talking about the jetpack frame and harness, and its primary purpose is to hold the jetpack indirectly on the back plate:
It was mass produced and the one on the Supertrooper suit was just a mock-up (or placeholder) and was never intended to use. It would be stupid to wear a Star Wars suit with a Star Wars merchandise (it's like the scene from the Space Balls when Yogurt shows all the products with the 'Space Balls' inscription - the movie was still running but they already had the merchandising campaign finished).
there is one super trooper that was recently approved in the 501st. it's an incredible costume, too. though he has the towel he got approved with a plain white cape, the star wars towel breaks the suspension of disbelief of the costume.