These are build plate guides for the Precise Printer Parts Ultra Flat Cast Aluminum Bed made for the Ender 3. I used this bed on my Ender3 S1 Pro with a Gulfcoast Robitics 3-Point Y-carriage plate. These simply provide a backstop for the build plate.
Best to print with ABS, PETG, ASA, or any other high-temp filament if you have a heated bed to prevent warping.
The author marked this model as their own original creation.