Folks, it seems that something needs clarifying here. 531 DB tubing was available in Imperial and Metric diameters. This is why a standard-gauge French 531 DB frame takes a 26.6 diameter seat post, whereas an English one requires a 27.2 post. The wall thicknesses are the same, but the O.D. of the tube is 0.6 mm different (28.0 vs 28.6).
Question: What sort of 531 takes a 26.4mm post? I've got an apparently French made 531 Bartali frame with a 26.4 and this seems by no means an uncommon post diameter for French 531 bikes.
Kurt Sperry Bellingham WA
-- fineartscrimshaw.com
531 DB (Metric) can be 26.4 or 26.6. Depends on gauge, and/or how much distortion occurred during brazing of the seat cluster (and whether it was reamed afterwards or not...). Greg Parker still in Madison, Wisconsin