This is the key required to open/close hydrants embedded in the exterior walls of commercial buildings. The key fits a 3/8 inch square valve stem. The socket fits a standard 3/8 inch drive.

When printed using PLA this is extremely strong. Commercially available keys are make to fit both 1/4 and 3/8 inch stems. As a result, the key does not fully engage the stem and rounds its end. Commercially available square sockets have an internal shoulder that prevent complete engagement of the stem. This also rounds the end of the stem. These plastic tools fully engage the stem and cannot round it because they are weaker than the metal stem.