Yes, the six-speed freewheel and the sidepull brakes suggest late 1970s. I had one of the first Nishiki Competitions, a 1971 American Eage Semi=Pro. Mine had a heavy, dead feel to it, despite the Ishiwata double-butted CrMo main triangle. Yours probably rides considerably better than mine did.
The Campag. hubs and headset are probably aftermarket upgrades.
If the bike fits you properly, I think you did well.
Your Compe presumably ranks pretty high in the Nishiki lineup. In 1972, they had the Competition (double-butted CrMo main triangle, clinchers, wide-range gears) and the Road Compe (full CrMo, tubulars, 54-44/14-24 gearing).
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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