This is a cam latch, commonly used for latching boxes. The design uses M4 screw holes with 15mm spacing across each latch, and 20mm spacing between the set of holes for each latch half.

23/10/17: Minor update: Added a chamfer to the outermost pivot to try and prevent the first layers sticking. Everything else seems to separate nicely, but the first layer is occasionally difficult.

03/06/18: Major update (V2 variants). As I wanted to use this design for a substantially larger box, I've rebuilt it, with increased tolerances between each part, a stronger cam action, and thicker arms for extra strength. Tolerances between plates are now 0.4mm, and between the pivots are 0.2mm. I've also positioned the camlock arms to have minimum overlap to prevent the plates seizing together. If it doesn't unstick after printing, then check your printer flowrates. I've recently had to drop mine to 0.9% in Cura. In terms of difficulty, I printed this quite comfortably on an old and well used Wanhao i3 Plus without problems once I sorted my flow rate. I recommend this test to check printer accuracy. https://www.thingiverse.com/thing:1622868