You are correct, the binder at the uppermost part is the 33.9mm tube slides over the top of the tube, which is reduced to receive standard 27.2mm seatpost.
Originally Posted by
wayold
1.375" tubing (with various wall thicknesses and almost any length) is commonly available and is ~1 mm thicker than your desired 33.9mm. I would think any machine shop (or even an amateur with a lathe in their garage/basement) could turn it down to size. I don't know how the binder attaches, but if it's just a reduced diameter section at the top of the tube with a longitudinal slit cut in it, that too is easy for a shop to do. Of course, even though a machinist could do this in a few minutes, it would probably cost a pretty penny from a professional machine shop.