Welcome to Club Carlton! I am a very new member, myself -- my Franco-Suisse is my most recent acquisition / project bike. This is the first good English road bike I have ever owned, after a couple of gaspipe converted 3-speeds during my undergrad days at UCLA.
I can confirm the 26.4mm seat post diameter for a plain gauge 531 frame of roughly that vintage -- my 1959 Capo. The Raleigh Super Course remains a highly sought-after frame.
On a frame with a relaxed geometry and long wheelbase, about the only benefit of tube butting is weight, and it does come with increased frame flex. My first Capo, a 1960 Modell Campagnolo with double butted tubing, was pretty whippy/flexy when I loaded the Pletscher mousetrap rack with textbooks. (Older readers will remember those.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069