I've got a mystery frame that has an old 531 sticker on it. So far I've been through 27.2, 27, and 26.8 and none of them have fit so far. The 26.4mm seems to, but I haven't tried to tighten it yet. I didn't even realise there was a 26.6mm seatpost. What a faff.
Missed this... I had a 531 (main triangle) Peugeot PKN frame that definitely took a 26.4 post.
Missed this... I had a 531 (main triangle) Peugeot PKN frame that definitely took a 26.4 post.