The Stovebolt Site Forums The Shop Area Engine Shop GMC 228 - What is this part called?

I had one of those on my '40 (216) my impression was it was a choke in the manifold ( to maybe help prevent overloading the engine under heavy work ) ???

I notice the car engine is listed as 85 HP and the truck at 78 HP , same engine . I never reinstalled it , I like the way the truck runs at its best .