This velocity stack has only a single, minimally designed fin to support the center cone. It doesn't seek to direct the air, but rather to allow the swirl of turbulent air as much as possible on the way through.
The gradual curve of the outside lip and cone gradually reduce the cross-sectional volume down to only the width of the actual fan blades. I haven't done any flow modeling, but I noticed an anecdotal increase in fan output and decrease in fan noise when adding this.
You'll need M3 screws to attach this. The holes in the mounting surface are 4mm deep, so be careful to measure the thickness of your fan mounting surface and not put too long of a screw into this part, or you'll either not get it tight enough, or blow through the outside. No M3 nuts are required.
You may also like my other related listings:
Velocity stack, 5015 blower fan
Fan duct, 40mm, 65 degrees