I had a Reynolds 531 track frame pass thru my hands that took a 26.4 seatpost. I guessed that the seat tube actually was straight gauge tubing, and as this was a big size, it might have been "best practice" to use that for a big track frame. I would guess that a Viner would be Columbus, and maybe has a straight-gauge Columbus seat tube? But there are cases of top Italian builders (Masi, Cinelli) building with 531, and Cinelli typically brazed an insert into the (Columbus) seat tube that reduced the ID and required a smaller diameter post, I think 26.4 was typical for them (experts will correct me I'm sure). I'd be surprised if a Viner is made with either Japanese brand tubing...given the JIS parts: the BB could be from a Sugino 75, not a cheap crank at all, but the headset sounds like a cheepo Miche rebranded...cheap but Italian.