The area in the end of the cylinder that has what looks like a wide saw cut part way around the OD of the tube is grooved all the way around the ID. The gland nut has a similar grove around the OD with a hole drilled in the bottom of the grove at one point. After the nut is put in the end of the tube a piece of square stock with a hook bent in the end is inserted in the notch in the tube, the hook goes in the hole drilled in the nut. You then turn the nut which pulls the square stock into the grooves, locking the nut into the tube.
As far as what it is stuck on there is an o ring on the nut after the groove. When you pull the nut out it gets stuck in the groove cut into the ID of the tube, you can see it in the slit in the tube. I know it doesn’t seem like an o ring would hold it up but it does, and can be a bear to get out. I think the last one I did I cut the o ring and pulled it out of the slit in the tube with some forceps.
One other thing, the sliders that wrap around the I beam wear out this then can put a bending load on the rod end and breaks the rod end off right where the threads meet the unthreaded portion of same.