This is a gridfinity crate for standard 9x11 inch sandpaper (Folded in half, hamburger style). Gridfinity is a system developed by Zack Freedman.

It takes a lot of filament and is pretty stout intentionally as it's designed to be used and moved around in a shop. This unit is 16U not including the little handle. It is not stackable and does not have magnets. It is 6 units long and 2 units wide. 

Happy printing!