This is a Hakko B2972 Control Card for the Hakko soldering and hot air stations. When inserted into the soldering station it allows for the tip temperature to be changed, and locks the temperature when removed.
Tested with the FX-951 soldering station, but should also work with the FM-203, FM-204, FM-205 soldering stations or the FR-802 and FR-803 hot air station as well.
Useful if you lose your original control card and have the iron stuck at 900 degrees (like I did). Granted, a popsicle stick would work too, but you probably wouldn't be here if you didn't like 3D printing things, am I right?
The Hakko devices use this key to block an light bridge (optocoupler) which signals the CPU to allow the temperature to be changed. Darker colors usually work best but depending on your filament white may work as well.
Increasing Reliability with Lighter Color Prints
If your key doesn't register, dipping the end in paint or coloring it as shown can help block the light bridge enough to trigger the temperature change mode.