Interesting, I used to work for a park bench & seat manufacturer. The legs were round steel tube, with either a flange to be rawled into the ground, or longer legs that were sunk into the ground and concreted round. They were attached to the seat frames by sliding over smaller dia tube and a bolt through. Naturally they were seamless tube, but the steel supplier once sent a big batch of seamed tube by mistake, we couldn't get the right stuff in the lead time needed, so ended up grinding the seam down with a die grinder and stones, took ages and was a very boring (no pun intended) job. Our machinist did try mounting them in the big lathe and boring them out, but it took longer than the die grinder. The welded slug broach might have been a better option, though we only had a fly press, so the all thread method would probably have been used.