There are a couple of other possibilities. If the owner wanted a micro-adjust post, they may have reamed it to 26.8mm, which is a very common size. Micro-adjust posts are available in the smaller sizes typical of hi-tensile steels but can sometimes be hard to find. Also, we can't discard that it may be a ultra-lightweight hi-tensile seat tube, even though I've never seen one that thin. Low 26mm range are fairly common on some club racers and the thinnest I'm aware of is Columbus Zeta, which used a 26.6mm post.