This is the HP .40 Carb Barrel used on most of the .40s.

The very first version of this carb used a swagged on throttle arm with a coil spring for the idle needle. Next they ysed a retaining nut with a coarse thread for the throttle arm. The threaded section was prone to breaking off if over tightened.
This was changed to a fine thread on the last and most common version.

Fine thread