This looks and sounds awesome!!!!!
The XM is the 7800 eXpansion Module, it plugs into the catridge slot on the 7800 and gives it better sound, more ram and has high score cartridge support built in. It will also allow to attach a keyboard to the 7800 for more advanced features.
This has been made to match the specifications of the one Atari planned to release for the 7800 back in the day that was scrapped due to the take over of Atari by the Tramiels.