An old style fly screen mount or retainer. These have been printed in PLA and have been in place for nearly two years.
The original version screws to the fly screen frame and has a spring loaded pin to hold the fly screen in place. The replacement version has the same profile; but just uses a screw or bolt instead of the pin.
The original version just isn't available anymore and it is far cheaper to make these 3D printed replacements instead of replacing the whole fly screen frame.