The takedown sleeves are seamless carbon steel. I welded in solid steel plugs and drilled and tapped one end, turned and single-point threaded the other end. All that work was done with my Logan metal lathe. I got interested in that method after seeing a photo of one made in the 1920s or '30s by James Duff.
I have since come to prefer a single sleeve that the limbs are just epoxied into with on end made removable by waxing the inside of that end of the sleeve before gluing the end in. The joint assembles and disassembles just like a fishing rod.
Here are a couple of images to show the difference.