Craig,
I have an Atax seatpost on my 70's Cilo. I don't know if it was original since the bike also came with Campy Victory components, obviously added much later. It's fluted and has a tightening arrangement much like that on a stem, the long allen bolt runs through the post, threads into a wedge and expands as you tighten it. I've also seen one advertised on the Cyclart website. It's a very nice looking post that has a gentle curve as it reaches the seat mount, much like the new Campy Record posts. It may not be period correct for your application, but it would look nice on just about any vintage bike. Mine is a 26.8 diameter. Hope this helps.
Fred Durrette
Summerville, SC