Saw this posted by the designer on the "3D Printing Useful Things!" Facebook group:
It's called a "screw sizer" and basically lets you measure or quickly identify the random screws and nuts and bolts you have lying in a drawer or box in your shed.
I used use a digital calipers bit this seems much faster!
The horizontal part is a ruler which is tricky to see in that photo.
There's a few versions of it including a metic and imperial one and some others.