As the prices of home computers like the Commodore 64 and Atari 800XL began creeping down to the sub-$200 territory, Atari realized that in order to lower the price of the 5200, some serious cost-cutting in the production of the machine was necessary. Since the 5200 was one HULK of a machine, it was only logical, then, to take out all of the excess metal and plastic that was only there for cosmetic purposes. What you see here was dubbed the 5100, or "5200jr." as we like to call it.
Much of the downsizing came in the way of eliminating the joystick storage bin above the cartridge slot. The original production 5200 unit had a handy area where you can flip the lid and store a pair of joysticks. As is the case with the 2-port 5200 model, the 5100 did away with the automatic RF switchbox and the funky power supply that plugs into the switchbox instead of the 5200 unit itself. Had the videogame market in the early 80s survived a little longer, there is very real possibility that the 5100 would have been released.