Per Low End Mac, the "original" Macintosh (later "Macintosh 128K",) was M0001.

The 512K was M0001W.

The Mac Plus was M0001A.

The 512Ke was M0001E (or D for the educational model.)

Only upon the introduction of the SE and II was a Mac anything other than "M0001", potentially with a letter on the end.