You cannot determine the OS version from the serial number alone. To check OS:
Korg issued serial numbers sequentially from the start of production in late 1987 (shipping in early 1988) until the M1 was discontinued around 1995. Korg M1 Serial Number
These are the last M1s ever made. They have the best keybed action, updated operating system 2.0 (supports more RAM), and typically came with the "M1R" expansion board. You cannot determine the OS version from the