After some time out in the islands, I am finally getting around to giving this some more thought, as I am rebedding deck hardware and deciding whether to remove these. They are quite beefy, so seem useful for something. But, I am not using them, currently. Took a better picture. I am backpedalling a bit about my assertion that they are solid all the way through (rather than welded tubing). Closer inspection reveals that an end cap
might have been welded and carefully ground down and polished. If so, they did a nice job with no clear evidence it was ever welded. In any case, it feels completely solid and clanging on it with a wrench sounds solid though inconclusive. If not solid, it is made of thick-walled tubing.
I am well-convinced that no part of the assembly is meant to be unscrewed. It is also clear they are not original equipment because the deck has been repainted at least once and this was installed after it was last repainted. I have maintenance logs going back to the original owner in 1976, but no mention of these.
In the groove, there is some evidence that something used to ride in that groove, as there are some wear indication in there - not a lot, but some. But, if you look above and below the groove, you can see some wear scratches that clearly show something has been rotated around the assembly and rubbing outside of the groove. The odd thing (and perhaps an important clue) is that those scratches are not perpendicular to the groove. They sort of spiral up (or down). Of course, I neglected to note if the scratches on the port-side spiral in the same or opposite direction. I'll look when I'm back out there in the next day or two.
If I can't figure out what they are and/or I don't need them for their original purpose, I am trying to come up with a clever secondary usage. They are so solidly built and installed, I kind of hate to just remove them.