A 28.6 clamp worked just fine on the front derailleur. I then thought the frame may have been repainted and a new sticker applied but it certainly all looks stock. I did check for the helical ridges and can feel some however they do not seem very deep.
I have never heard of this metric Columbus, any info would be appreciated.
Thanks
WjV
I don't have much to add: I've *heard* that Columbus made "metric" dimensioned tubing, similar to the much more common metric 531 used by so many French builders...but I've never seen any of the Columbus tubing in person. The only French bike I have personally seen that was built with Columbus (a Motobecane) had a 27.2 seatpost, IIRC.
The helical ridges are pretty subtle, but they are raised, not incised and maybe only .5mm or so...should be 5 of them and SLX should have them at the butted ends of the main tubes, so you'd see and feel them in the open BB shell.
I can't see what the seatcluster is: there's a chance Cilo may have brazed in a thin reinforcement to the inside binder clamp area which would reduce the ID. But there's also a chance of a mixed tubeset (with a plain gauge or heavier weight seat tube) or even an up-side down seat tube (not likely, but it HAS happened!)