This Holley HP carburetor was bought with our current motor, we have never used it.
Here is some good information from Advanced Product Design about the Max Speed throttle stop.
THIS NEW DESIGN MOUNTS THE THROTTLE STOP WITHIN THE CARBURETOR’S LINKAGE AND HAS A VERY POSITIVE STOP ON BOTH OPENING AND CLOSING. WE HAVE ALWAYS FELT IT IS BETTER TO SHUT THE CARBURETOR ITSELF THAN TO SHUT OFF THE AIR BELOW IT. WHEN AN UNDER THE CARB STOP IS USED, IT SHUTS OFF THE SIGNAL TO THE WHOLE CARBURETOR. AN INLINE STOP SHUTS THE CARBURETOR BLADES, AND STILL KEEPS THE SIGNAL ON THE IDLE AND INTERMEDIATE CIRCUITS ALONG WITH THE ACCELERATOR PUMPS TO HELP THE MOTOR RECOVER WHEN COMING OFF THE STOP. WITH THE DESIGNS OF THE PAST INLINE THROTTLE STOPS, INCONSISTENCIES IN STOP RPM AND E.T. CAME FROM THE PEDAL STOP INSIDE THE CAR ALONG WITH CABLE MOVEMENT AND FLEX. THIS NEW DESIGN FIXES THOSE PROBLEMS. THE BRONZE THROTTLE LEVER THAT YOUR CABLE ATTACHES TO GOES AGAINST A WIDE OPEN TOOL STEEL STOP, AND NEVER NEEDS TO MOVE TO GO ON THE STOP. ALL OF THE LINKAGE PIECES ARE CNC MACHINED FROM TOOL STEEL AND BEARING BRONZE TO EXACT TOLERANCES.