The importance of the seat tube outer diameter is that French and Swiss bikes tend to be hard metric (28.0mm), while everyone else's frames are English (28.6mm, which is 1-1/8").
A seat post diameter of 26.2mm or smaller with an OD of 28.6mm would definitely indicate a straight gauge frame, probably plain carbon steel, rather than a higher-strength moly steel.
The width of the bottom bracket shell often, but not always, separates Italian frames (70mm) from most others (68mm).
The best help in establishing country of origin is the bottom bracket threading.
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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