This is for an APP 50 paintball hopper. It helps keep the lid from popping open.
The narrow side is made to interfere with the hinge and push the lid up and back, allowing the latch to hold better and not pop open from blow back, while still being easy to open. It also helps to hold the mouth of hopper in more of a circle, as they can get a little squished. The rise in the sides helps lift the center of the lid to try to add a little bow upward.
Tested on a pump mag with a good bit of blow back.