This 3D printed cartridge case is for the Atari Ultimate cartridge. This is an SD based cartridge interface that allows programs to be loaded off SD card as though it were connected to one of the old legacy drives. Similar to the 1541 Ultimate device for the Commodore.
Please take note that this is for the cartridge case only. You will need to obtain one of these devices for the case yourself. They are available from time to time on http://atariage.com/forums.
The case is available in a variety of colors.