This is a mounting platform and sign bracket for the Hughes XE11 kit card by Nakozen available at https://www.printables.com/prints/143198
These parts are designed to attach to a 3UDC pivot arm so that you can present this model anyplace you have a 3UDC socket.
You can find the Kit Card Tree library of mounting hardware and instructions at https://www.printables.com/prints/126495
Thanks to Nakozen for sharing this print.
The author remixed this model.