15 horse, "C" model (15 c) - 100% Yamaha built (for Merc), for sale primarily in Asia and parts of Europe. The popularity of some particular models saw them come to North America as a lower cost alternative to (real) Merc motors. These were not "cheap" or "junk", simply motors based on technology that was a few years old, so they didn't have the "latest" engineering or the bells/whistles of either Merc or Yamaha motors of the same model year.
684 - Yamaha designator for their model 15 B which began production in 1978 (so regardless of the year of production, the motor is "mechanically" a 1978 Yamaha model 15 B).
C - originally mfg for a control box (remote) but tiller equipped
S - standard (short shaft 15")
005453 - the serial number indicating a "manual start" with a production date in 1979
For the purpose of parts if you are looking up on a Merc site you have a model "15C" with a type/serial of "M-684-005453