Series 9 fans have a ring or shroud cast around the blade tips which confines and directs the air flow. This type fan may direct the air through or over the motor without the benefit of a close fitting fan cover. The ring also provides a stronger construction, limiting the damage to the blade tips. (A support ring could be cast along the blades of most patterns.)
Select the Overall Diameter Range (D) that the fan you are looking for fits into.