How do you think this stairway was built?

This gorgeous spiral staircase is from the lighthouse at Sand Island, Wisconsin, leading from the basement fuel room to the lighthouse room at the top of the house, with stops at the two floors of living space along the way. Our tour guide told us that nobody knows how it was built. Some people, he said, think each section of the metal stair fit together with a male/female sort of locking mechanism. Others think that a pole was first installed and each section of the metal stair was slid down that. Either way, the staircase is mostly held in place by tension—there's nothing connecting it to the wall at all, except at the two landings in front of the doors to the living quarters.

The trouble with being married to an engineer: I just assume that all "mystery stairs" are only really mysteries because the right experts haven't ever been called into evaluate them. (I'm lookin' at you, Chapel of Loretto.) The fun thing about being married to an engineer: Once you get past "it's just a mystery, I guess!", then you're left with a cool problem-solving puzzle to hypothesize about.

So what do you think? I'm guessing this is almost more of a history question than an engineering question, because part of what we're trying to figure out is how sections of a spiral staircase like this would have been joined together back in the day. But the other part is pretty engineer-y: Do you think the brackets (they're about iPhone sized and there's two under each of the two landings, which are both on the South side of the stairwell) are the only thing supporting this staircase other than tension? Or do you suspect that it's held in place some other way?

I'm going to be traveling until the 9th. But, when I get home, I'll check around and see how close you all got to a correct assessment.

This is what the landings in front of the doors to the living quarters look like. The edge of the sections right in front of the door fit into brackets attached to the wall underneath.

Here, you can see how the sections of staircase sit on top of one another. There might be a pole in the middle. Or each section might have been made to fit together. You can see how each of the sections are attached to one another with a bolt at the edge, but they aren't attached to the wall.

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