Originally Posted by
jliet
Nope, I was just wrong. What I assumed to be a shim without looking is actually the seat tube and the mitred part is coming apart a bit.
I expanded it a little with a screwdriver, but a 26.2 seat post enters the enlarged section, but won't go deeper than 2-3cm. So, indeed, it seems it's a 26.0mm seatpost... No idea how that works tubing wise.
You said it was a 28.0 OD, so you have 1mm (or maybe 0.9mm) thick walls - not butted probably, but still possibly good tubing. Strip it and weigh it.