Which aero seatpost fits this frame?

[darkhorse75]

Picked up this frame from gumtree for $10 a while back and I'm a bit stumped as to which seatpost is suitable for it. The frame is made from ovalized steel tubing the only identifying marks being a Cicli Stelvio headtube sticker it also came with a shimano 600ax headset and shimano aero stem. THe bottom bracket is english thread with shimano dropouts front and rear and seems to be made for 27" wheels.

Thinking it was Tange Aero tubing I bought a Shimano 600 ax seatpost off ebay recently but it's not a very good fit at all. (photos below) The grub screw at the rear of the seat cluster doesn't even reach the post when fully screwed in. The seatpost hole measures 32 x 20mm and the seatpost measures 29 x 18.5mm. According to Sheldon there were two versions of this seatpost and the other one is 34x 18mm which won't fit either.

This got me thinking it could be Columbus air. There is a Columbus air seatpost on the bay at the moment which I asked the seller to measure and he said it was 31.5 x 16.5 max width. This seems to be the right length for the oval cross section but the width is all wrong. I am stumped, if anyone can shed some light I would appreciate it.

Thanks,

Chris.