C-mount and CS-mount lens tester box

There's two widely used standards for small lenses, C mount and CS mount. They can both screw into the same socket, and it can be tricky to figure out which is which.
If you get it wrong you'll end up with a picture that'll never be in focus, no matter how you try and twist it.
This box allows you to check your lenses in under 5 seconds. Just hold the lens up to a hole, and see if an image is formed on the plate. If not, flip the box upside-down and try again.