Some of that bathrail tubing is made from a metal strip thats been rolled into a tube. Its got a seam down the side of it.
They, as the roll the tube, add a long shim of brass to it, as the tube is rolled the brass shim is on the outside & its gets rolled as well, as the tube is made, the shim is drawn around the tube & as the slit closes, the edges of the brass shim get caught into the gap.
Then the tubes rolled closed & the shim is covering the tube & the ends of the shim are caught tight in the slit, the tube is finished & covered in the shim.
I have seen it a few times, real old stuff, look down the inside of the tube & you can see the edges of the shim. Or just bend it & the slit twists open & the tube collapses & the brass shim lifts off.
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