These special fasteners allow you to joint tubes together using screws instead of plastic rivets. This is a stronger joint, and it great for holding the e-bay in your rocket. Specially treated so that epoxy will stick permanently to the aluminum inserts.
There was a time when you could buy them straight from the manufacturer and I got a whole bunch of them in bulk. Now, it appears you can only get them from Apogee. Too bad, they are pretty slick and Apogee is selling them at a premium.
Once my current supply is gone, I am not sure I will buy them at $5 a pair. I have been looking around for a replacement, but have not found one yet.