Part of an assembly suite originally designed to work with 3D printable Skadis boards.  These simple clips are an extended version of the original ones made for the bottom of the assembly to provide extra support against the table.  The overall assembly is a bit tricky to put together but once completed the boards are evenly spaced and able to have a good support.
 

Parts for the Assembly

Four way center connectors

Skadis Clips

Extended Skadis Clips (this model)

3D printable Skadis boards by AU3D

Remixed Skadis Stand by CMLab