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Problem with my Merckx was someone had tightened the seat clamp , without a post in it , and all of the rubbing was coming from the top of the seat cluster as you have kinda alluded to. I fixed mine by levering with a big screwdriver , but the smart fix is to put a bolt through and have it push against a flat washer inside of the seat cluster slot if that makes sense , will need a slim nut on the bolt also inside the gap. So effectively you are able expanding it ..
Jeromeo-
The scarring is coming from the peak of the scooped seat tube directly opposite the clamp slot. The bolt was nipped up when i went to open it for the first time but not that tight. The slot still looks parallel and its quite long with the bolt at the very top. I did leave a post in overnight hoping that may open it up a bit but it came out this morning with as much difficulty as it went in. I'll get proper calipers on it at the weekend so that'll show if it is pinched. I say it looks parallel but i can't eyeball 1/5 of a mm.
Retro_Bastard
It must need a little bit removing all the way round as the corrosion is all the way round making it tight but the bit sticking out causing the scarring on the post. I was hoping if i can find that bit and remove with a round file then polish the rest i should be good. But I've decided I'm not going to spend all day on it. It really shouldn't take long to remove the surface corrosion so the longer i spend on it the closer I'll be to fucking it up so i may be calling for pro assistance next week.
EDIT I've just inspected the tube again and had a post in the top and the scarring on the post isn't coming from anything inside the tube it's being caused by the tip at the front of the top of the seat tube. I am coming round to the idea that i may be being talked off a cliff edge here. How many people have said I've never done it before but how hard can it be? Then fuck it up massively