Hello! This auction is for one of the very rare Atari 1090 XL Expansion System units developed by Atari in 1983 for the 600XL and 800XL home computers, to enable them to use up to 4 expansion cards. It was fully developed and Atari produced ~50 chassis and PCB's and after they dropped the product before its release, these 50 prototype units were liquidated at Best Electronics in the 90's. However, these were sold without the beige top cover, and only 5 of these were ever produced. More information on the history here. 

Judging by the "C.E.S. #4" written in indelible marker on the bottom of the chassis, I guess that this unit was displayed at the Central Electronics Show.

I don't have the power supply or PBI cable to test it and I want to leave all of the original caps and chips intact, but it came from the collection of a big Atari fan who kept it stored well. The metal and plastic parts of the case have minimal blemishing with a few minor scratches, and the item is exactly as you can see in the pictures. When I got it, it was missing two of its four rubber feet, so I replaced the two missing ones with a very similar type of black rubber foot so it no longer wobbles, and you can barely tell they're different.

I will ship to the US for free within 1 day, request payment as soon as possible after the end of the auction, and will gladly ship internationally for $20 extra. 

Thanks for looking and let me know if you have any questions or comments!