The Mac 512k also unofficially known as the "Fat Mac" is the Direct successor of the [wiki]Mac 128k[/wiki]. The 512k was created due to the insufficient 128 kilobytes of memory in the original Macintosh. Other than the expanded memory, there is no difference between the Mac 128K and the 512K, as they share a logic board that had been redesigned from the original logic board of the original Macintosh.
Later, the 512Ke was released which included the same ROM as the Mac Plus, and which shipped with an 800K double sided floppy disk drive.