It seems to me that the French 531 Bartali owned by Kurt Sperry, must be a plain gauge tubing frame, if it has a 26.4 mm internal diameter, although Sutherland's guide gives this as an alternative to 26.6 for a double-butted tube. 26.2 is given as the PG internal diameter.
Norris Lockley Settle UK but packing my bags for a further 7 weeks in France..and maybe a glimpse of the Tour-de-France...but it doesn't grab me like it used to do in the days of the cotton casquettes and swooping external non-aero brake cables...