Beretta's OEM Skeet chokes work just fine in the 391's, whether Mobil or Optima Versions, flush or extended.
I would stay away from ported or vented chokes entirely.
I much prefer the extended versions ... like the Mobil "Victory" series or the colour coded Optimas.
Much easier to change (no tools required) long enough to get a firm grip on them to brush out any crud & plastic fouling, and the colour-coding tells you at
a quick glance what choke is in place. For Skeet you might prefer the blue banded Cylinder choke, the red-banded Skeet or ( somewhat tougher to find )
the purple-banded American Skeet. Nominally, Cylinder is no constriction, Skeet is .003" and American Skeet .007". All three will give decent breaks at 21 yards
however, the .007" chokes are noticeably tighter without being over-choked.