This is a cap for the Kuretake Waterbrush Petit that allows you to transport it with water inside,so you don't have to carry along the bottle of water that comes with it. With this small addition it is now the most portable waterbrush I have been able to find, and fits quite nicely in an Altoids-sized tin.
I printed the model in the pictures on a Makerbot Replicator II (flip the model to flat side down for printing). It's meant to come off just a tiny bit more easily than the brush comes out since you'll probably take them off in that order.
Note: If you squeeze the brush hard while the cap is on, water will still come out the edges, so I wouldn't store it somewhere it's going to be compressed.
I bought the specific waterbrush from JetPens: http://www.jetpens.com/Kuretake-Waterbrush-Petit-Set/pd/2674