I've been following the pen-making board on the Fountain Pen Network for a while now, trying to learn as much as I can before making mistakes on the lathe.
From what I've seen, if a cap has a clip then it will be made in two pieces -- the cap and a finial -- and the ring of the clip sandwiched between them just like a kit.
Some makers hide the ring by making the diameter of the cap/finial larger than the diameter of the ring, and cut a rebate into the material to enclose the metal ring.
I've seen some threads saying that the diameter of the clip ring was too small for the cap being made; that maker chose to cut the ring away, leaving lugs as you describe, but he still made the cap with a finial that screwed in and bent the lugs around the tenon before screwing the finial down tight.
Here's one post that shows pix of all the pieces as well as the assembled pen. There are some great pens posted in that forum.
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