This is an under the desk mounting bracket for Dell Micro form factor computers. Specifically in my case the 3046.
This has 2 brackets, made to go around the computer at the rubber feet. So if yours are missing, you may need to remodel it for that.
The rear mount has a stop for the computer to hit that way it can't fall out the back if you are to push the power button, or plug in a flash drive.
The author hasn't provided the model origin yet.