My single data point, for what it's worth:
In March 1971, I bought a brand-new American Eagle Semi-Pro [Nishiki Competition], #KS78091. Sorry, I don't recall the serial numbers of the various Nishikis we sold at Bikecology, although I think many of them did begin with the same two-letter sequence as mine.
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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