On September 10, 1984, Apple shipped the Macintosh 512K, aka the "Fat Mac." It was the first upgrade to the original Macintosh 128K.
(via Cult of Mac - Your source for the latest Apple news, rumors, analysis, reviews, how-tos and deals.)
Cult of Mac
Today in Apple history: Macintosh 512K, aka the ‘Fat Mac,’ quadruples the memory
Shown as published in the source's RSS feed. IndiaAIFounders doesn't link out to third-party sites — this is the complete detail available from the feed itself.