Here's an explanation of the stepper, by colintornado - benelliforum - April 2007

"What is described as the "stepper", is the idle air control valve - it is a stepper motor. It moves in set increments, by receiving inputs from the electronic control module. The motor moves a rod in and out to open and shut the airflow from the airbox, to the throttle bodies, below the butterfly valves. The ecm is set to idle at, say 1200rpm, and it uses inputs from several sensors, and then regulates the stepper, fuel injection and ignition.
If the stepper fails, it causes rich, and poor running. If you look in the airbox with the top removed, the stepper can be seen on right side. If the engine is run, the stepper can be seen to move, if reved up and down. You can see a white plunger that uncovers the little ports to the outlet pipes.
Tuneboy has a test for the stepper, not sure if axone can do this, both may show fault code for stepper failure."

Alternatives
There are a few alternative IACVs that have been used successfully. Apparently that from a Triumph 955i is a direct fit, as Rick67 discovered.
UpNorthWill also reports that the stepper from a Triumph 955i is the same part. "they are plentiful and about 5x cheaper."

The Triumph part is T1240888. It fits TT600, Tiger 855 & 955, Sprint RS and ST, also Daytona 955i and 595.

Ilija (ilijasrb) found 3 parts from Alfa Romeo, Fiat, Lancia & Peugeot that could be adapted. Some of these have different connector angles than the Benelli part, but all will work. All that needs to be done is to remove the piston and install the Benelli piston, as they are a different shape. This must be done by running the stepper motor. The piston can not be screwed out as there is a plastic guide that stops it from spinning. He describes how this can be done in his post. I use a stepper driver that I sourced off ebay.
The part numbers are:-
9945635
7700102539
& 95 644 484 80 1920.N1 (you'll have to do a search for that one, and maybe all if the links are broken)
These car IACVs are a deal cheaper than the OEM or Triumph parts.