This is a rotary switch that is used to hold down the momentary power button on inexpensive lasers that you can find online.  I bought mine on eBay for $10. This works with the one shown in the photos and measures 22.5mm in diameter.

Slip it over the end and it snaps in place over the button. Twist to power on the laser and twist in other direction to power off. I use this laser to polar align my equatorial mount and need both hands to adjust the alignment screws, so this keeps the laser on during this operation.

I printed this with organic supports.