This is an interfacing plate to mount a Connect Systems CS800/CS800D control head. The plate interfaces with the control head mount and provides screw holes. This enables the control head mount to be attached to a smaller mounting point.
I did use a 3D extruder pen to attach the interfacing plate to the control head mount in place of screws.
You can find out more on the mount at: https://08milluz.com/2017/05/31/mounting-bracket-for-cs800d-dmr-amateur-radio/