This is the future rear sight for Benelli instead of the fixed ones.
Adjustable from 2.3 to 4.0 mm in 10 steps.
Further more you can lift the 'floor' of the sight to keep the picture square.
Require minor change to the grip.
Many have trouble with a sharp sight picture, and can benefit from a larger opening.
PLEASE NOTE: These fixed sights are no longer in production, why the 4.0 mm is not available any more, and when the 3.3 and 3.7 are gone it's over.
Instead the new adjustable sight is finally available, covering 2.3 to 4.0 mm - please check that out.