Copied from a similar thread on AR15.com:
What you have there is a factory-made M1 Benelli barrel ordered by Benelli USA for use in the Entry Model, but never actually cut to 14”.
NFA firearms are not importable for civilian sale, so as a workaround, Benelli USA orders and imports from Benelli ITA as an 18” model with the sights installed for 14” length, just as you observed, then cuts to 14” and Form 2 registers as an NFA SBS.
Never seen one make it out of the factory, pretty cool.
ETA: Benelli doesn’t really position their NFA shotguns for civilian sales per se and the Entry models could be imported directly as NFA guns, however there is a ton of additional paperwork involved and the imported NFA guns become the practical equivalent of a pre-may dealer sample MG. This workaround is perfectly legal, smooths the distribution workflow and allows the resulting gun to be transferable rather than only available to LE and SOTs.
That barrel is certainly an oddity to most people, but is worth a premium to a Benelli M1/M2 owner looking to Form 1 their gun into an SBS.