This thing is an Arca-Swiss compatible camera mounting plate that is similar in form to the Peak Design Capture plate.

Being a big fan of Peak Design, I have some small lightweight items that would be great to be able to use with their Capture clip so I drew this part up from the Arca-Swiss standards. There are four lanyard loops on the plate for attaching Peak Design anchors.

The mounting location is designed for an ultra-low-profile socket head screw (1/4-20) like this one: https://www.mcmaster.com/91223A216

If you are going to be using this plate with any valuable equipment, please consider purchasing the aluminum plates from Peak Design as they will be extremely secure, whereas this part comes with no warranty for performance.

I've also designed an Arca-Swiss compatible tripod mount that you can find here:https://www.thingiverse.com/thing:3949964