These are thumbscrews for collimating an SCT telescope's secondary mirror.  They're designed to fit a Celestron CPC 1100 (which uses 12 mm M3 screws), and probably fit many other Celestron SCTs.  They're large and easy to turn precisely, but can't be seen from the primary mirror, and don't interfere with the cover.

You can print these without support.  After printing, use a 20 mm M3 screw to pull one nut into the recess, and then jam a second nut on with a dab of thread locker.

When installing these on the telescope, make sure to only remove and replace one screw at a time, so you don't drop the secondary mirror.