This is an undershelf mount Support bracket to create a small 10" / 19" rack. It is an application of this Model https://www.printables.com/model/1150903-rack-server-rails-self-supporting-m6-m4-thread-col (with thanks to the creator of that model)
The .STL file is for the right side, just mirror the design in the slicer to create the left side version.
I printed it in Fiberlogy Easy PLA black and is it way strong enough for this application.
The author marked this model as their own original creation.