This adapter lets you use a 50mm x 15mm blower style fan on an E3D v6 hot end. The standard 30mm fan that comes with the v6 is very noisy. Blower fans push much more air and are so quiet you can hardly tell they are running.
The screw hole will fit an M3 x 25mm or 4-40 x 1" screw.
I have only test fit this on my Printrbot Simple. Probably will work with other printers but YMMV.
This is the blower fan I used: http://www.amazon.com/50mmx15mm-3500RPM-Brushless-Cooling-Blower/dp/B00UJBIAX4
In Simplify3D I placed supports under the arms. I was able to print this design without supports but got much nicer results with them.