The Commodore Amiga 500 Plus (often A500 Plus or simply A500+) is an enhanced version of the original Amiga 500 computer. It was notable for introducing new versions of Kickstart and Workbench, and for some minor improvements in the custom chips, known as the Enhanced Chip Set (or ECS).
The A500+ was released in several markets (including many European countries), but was never sold officially in the U.S.
Although officially introduced in 1992, some Amiga 500 Plus units had already been sold (masquerading as Amiga 500 models, and with no prior announcement) during late 1991. It has been speculated that Commodore had already sold out the remaining stocks of Amiga 500s, before the run up to the profitable Christmas sales period. In order to make enough A500s before Christmas, Commodore used stocks of the new 8A revision motherboards destined for the A500+. Many users were unaware that they were purchasing anything other than a standard Amiga 500. Although the Amiga 500+ was an improvement to the Amiga 500, it was minor. It was discontinued and replaced by the Amiga 600 in summer 1992, making it the shortest lived Amiga model.
Source: Wikipedia, "[en.wikipedia.org/wiki/Amiga_500_Plus Amiga 500 Plus]", available under the CC-BY-SA License.