I just purchased a beautiful Nishiki Custom Sport. The serial number in the bottom of the crank is S1 2B343. I think that its a 1972 model. It has centerpull brakes with Shimano 600 Arabesque derailleurs front and rear. Stem mounted shifters. Its a ten speed with 27 wheels. I believe the color is crimson red. Its heavy but rides beautifully. The foam grips are worn and torn and the front derailleur isn't shifting but I'll fix that. Doesn't have the original seat or tires, but it does have what I believe is the original rear rack, or at least a rear rack from that time period.

I read that the serial number beginning with S1 designates that it was made before 1975. And that the 2 is the year, B is maybe for February, and its the 343rd bicycle made that year.

I have pictures on my phone, I'll try to post at a later date.