As titled, this a drain plug for for the return header for a Raypak Pool Heater. If you need to tighten it beyond finger tight, it fits a 17mm socket, but do so carefully. May benefit from some PTFE tape or Silicon/PTFE lube.

Replacements on Amazon are reportedly made of Nylon 6, so that's great if you can print it. I elected to go with PETG as I'm too impatient to dry nylon and set up an enclosure/carbon filter. I printed with 100% infill for (probably imagined) additional strength, and 0.12mm layer height for (not imagined) improved definition on the relatively fine threads.

From the guy who keeps bringing you replacements for the weirdly specific things that break in my life. It aught to be quite the library some day.