This is a wall control peg board system retainer for the stacking box produced by HG_Design (https://makerworld.com/en/models/60223). There 4 different variants. The 2 thin variants require supports, where as the 2 thick variants does not use supports but instead is using snug connectors.

The 3 slots wide variant is to fit the 3 inch wide remix of the stacking box which can be found here: https://makerworld.com/en/models/646739

The 4 slots wide variant will fit the regular stacking box and any of my 4 inch wide sized variants.