The key parameters include the seat tube outer diameter (hard metric 28.0mm for French, 28.6 for most of the rest of the world), seat tube inner diameter (27.2mm with a 28.6 OD or 26.6 with a 28.0 is a good sign), BB shell width (not reliable, but 70mm tends to be Italian), BB threading (again unreliable, but 36x24 tends to be Italian). Serial number patterns sometimes help narrow down the field, as do lug geometries, as mentioned above.
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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