Howdy Chapa and All:
Take a close look at both carbs, side by side. You'll see the that the Auto trans carb has a mounting feature and a vacuum source on the side to mount the dashpot. The dashpot functions on vacuum. When the throttle is closed as coming to a stop, the engine is at high vacuum. That opens the throttle slightly so the engine doesn't stall out.
To confuse things, carb rebuilder ocassionally use automatic carbs for rebuilding as manual carbs because, typically, auto trans carbs have a tighter throttle shaft due to less wear.
BIll's suggestion is a good one. Give it a try and let us know how it went for you.
Adios, David