The OP's camera is a very late serial number Nikon MS (body engraving should read "Made in Japan" rather than "Made in Occupied Japan") and its definitely a repaint.
A true Nikon M in the eyes of collectors doesn't have flash sync but does have a serial number with an M engraved at the start. The Nikon MS, as its called by collectors, came next and its basically a Nikon S with flash sync and an M engraved in front of the serial number on the top plate. About 1,700 were made before Nikon ran out of top plates with the M engraving. The Nikon S which followed doesn't have any letters in the serial number.