An NS-code is atypical based on the Nishiki serial numbers that I've seen and I've seen my fair share. I'd expect a KS prefix. The Shimano 500 rear derailleur places it no older than 1975, provided it is OEM. The steel cottered crankset and brake safety levers are indicative of a entry level model, while the fork ends indicate upper entry level. It would appear to be an Olympic, circa 1975.
Edit: I don't recognize the shift levers but they look more Shimano than SunTour and the front derailleur looks like it could be a Shimano 50 ( please verify),so I'm assuming the rear derailleur is OEM, pending further information.
Last edited by T-Mar; 06-10-16 at 11:21 AM.