If you have a application where you want to use a standard 5015 blower to replace a 40mm box fan, these adapters are for you. There are 3 models. One is the most simple and is glued (Super Glue Fix-All or similar) to the blower. The second model has a provision to attach it to the fan with a M3 x 25 screw and nut, and the last one has a velocity stack, which may or may not increase air flow, just for grins.