That’s an overall picture of a riveted connection in a wrought-iron frame, with the central rib supporting a purlin on each side. The fun starts when you look at the sides, so you can see the rivets. First the left side:
Those don’t look like most people’s mental image of rivet heads. That shape was sometimes called a “pan head” – confusingly, there was another, similar shape also called a pan head – and sometimes called a “cone head.” Seriously.
Here’s the right side:
Those don’t look like our expectations either. Those are hand-hammered heads that approximate the shape called a flat head but not very closely.
In short, the rivets came to the site with the pan head on one end of an otherwise smooth shank, were heated red hot, put in place, and then hit with a hammer until (a) the second head was formed and (b) they cooled. The hand-riveting process was messy in every way possible, and those hand-made heads show it better than the symmetrical round heads made by a pneumatic hammer with a rivet-head attachment. As always, hand-craft reveals its process more clearly than machine-made.