Serial number 100,000 reportedly rolled off the production line.
While the serial number itself does not explicitly contain a date code (unlike some other Korg products where the first digits represent the year and month), you can estimate the date based on known milestones: Manufactured between 1988 and late 1990 . 100,001 – 250,000: Manufactured between 1991 and 1995 . Firmware Versions and the Serial Link
The Ultimate Guide to Korg M1 Serial Numbers: Identification and History