T-Mar you have very sharp eyes. After a cleaning of the bottom bracket you can just barely see a "K" which was not struck as deeply as the other letters and then filled in by the primer/paint.

This leaves us with a serial number of KS 17987, however I understand that the "S" is a designation of a pre-1975 Nishiki, but then we are left with one of two (?) interpretations of the 17987 number that follows, if the 1 is actually an "I" then the "I" would correspond to a year ending in 9, for 1969, however I think that the 1 is in fact a 1 and this is the 17,987th bike made, but what year? I believe the year to be either 1971 or 1972 and here's why.

The Sugino Mighty Competition cranks are dated 47-2 which would be year 1972 and month February.
The Sunshine Hubs are dated 5345, there is a circle stamp with some kind of symbol inside the circle before the hub number, and this symbol and hub number are the same for the front/rear hubs. The information I found on Vintage Trek is: "The "5345" marking on early Shushine hubs may be a model number. I (Skip) have Sunshine hubs from 1972, 73 and the late 70s that all have that [5345] number."
The Sun Tour front derailleur has a year code of OG which is 1972, month of July
The Sun Tour rear derailleur has a year code of OF which is 1972, month of June.

Given the above I believe this is either a 1971 or 1972 Nishiki Competition. If I have missed something or interpreted it wrong please let me know. Thanks

Oh and yes the Sugino Mighty Competition chain rings on this bike are 49t/42t as previously noted by John E