Eddy Merckx Columbus SLX

1. Seat post is 27.2mm
   
2. I can only feel 3 or 4 ridges inside the tubing but it's not spiraling if that's what you mean by rifling (sorry, I'm very novice at this matter)
   
   

I've got an SLX frame with this size seat post.
If you look down the forks it should look like the barrel of a gun
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcR6zzU-kepqGQkCfPn7PwJuYSOBSipGu9QE1kTjP8YZbJortnIQ
Like that. Although I think there's only four "spirals" on the slx tubes, not 100% though, could be more. Not every tube in the frame has to be rifled like this to be SLX (and I doubt it would be anyway), the inside of my frame is pretty rusty but I think it's only the forks which I can see the rifling in (and this bike has come straight from the manufacturer so I'm sure it is officially branded SLX). I'm guessing SL tubes are used when rifling isn't visible but this is still classed as SLX.

Weight should be the easiest way to tell what tubing it is, I've never felt a lighter steel frame than SLX.