My personal experience matches the "long answer," above. Some hubs designed for standard 6-speed freewheels accommodate 7-speeds just fine, whereas others need an additional washer on the drive side of the axle. A 6-speed with a pie plate spoke protector is generally pretty close to a 7-speed without one, but even then I have sometimes needed the extra washer to keep the chain off the right dropout.
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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