Might just be to indicate when it started happening. They could have written "M1" and still cause the same confusion, and I believed that's what the model is called.
"According to an investigation by one of the affected users, the Apple iMac screen uses a flex cable that must sustain a voltage of around 50 volts when the screen is set to high or maximum brightness. This causes the connector to burn out over time, it was theorized, resulting in short circuits that cause the black lines to appear on the screen"