The Model 500 telephone was originally developed and produced by Western Electric, the manufacturing arm of the Bell System. When it arrived on the scene, it took the industry by storm. Virtually all of Western Electric's competitors started scrambling to come up with competitive models. Most of them failed miserably to live up to the 500, so over the years, through various licensing agreements, almost every other telephone maker, including Northern Electric, ITT/Kellogg, and Stromberg-Carlson, would end up manufacturing nearly identical 500 sets of their own, eliminating almost all the variety that existed in years before. Automatic Electric would become the only telephone maker who never copied the 500 design directly. Instead, they made a set very similar to the 500, called the AE-80, using all of the design principles of the Western Electric 500, but using Automatic Electric's own parts and technology.
Cortelco was the final maker of the 500 to cease production. They finally ceased production in 2006, after producing it for over 52 years, making it one of the longest production runs of any phone in history.
I have more information to offer here on the history and production changes of the various brands of 500's:
For more information on the various model numbers in the 500 family, please visit this page on Paul Fassbender's site.
For more information on development and timeline photos of 500 development, please visit this page on Paul Fassbender's site.
For more information on COLORS, visit this page on Paul Fassbender's site.
You may contact me at zuperdeeNOSPAM@gmail.com, if you remove the part in caps. (I do this to try to prevent spam bots from mining my address.)