Following on a few months after the release of the original Macintosh in 1984 was the Macintosh 512K (M0001W). The 512K referring to the memory, a whole half a megabyte of RAM! The original Mac had just 128K of course so the 512K was quite an upgrade and indeed in 1984 was a lot of RAM for a personal computer. Apart from the memory the 512K was the same as the original Mac but the extra memory made it a much more useful computer.
Some new applications required the extra memory including Microsoft Excel which was first released for the Mac. The extra memory also enabled an early form of Mac multi-tasking and networking. The 512K remained on sale for 2 years.
The 512K is the oldest Mac in my collection. I use it as a stand for my bedroom clock. The Mac could still work though mine is a US 120v one so would need a converter to use power in this country. I do have a converter but the Mac (and converter) have not been turned on for a long time, i'd rather not risk blowing it up!