Flip the M1 around. On the back metal chassis, near the power cord inlet or the MIDI jacks, you will find a silver or black sticker. It will read "KORG INC." followed by a string of numbers. This is the primary serial number.
Numbers below 050,000 are likely from the first year of production (1988–1989). 3. Why the Serial Number Matters Korg M1 Serial Number
If you are referring to the software re-release (Korg Collection): Flip the M1 around
Format Example: 172981