This thing here is an official Nintendo DS flash cart. It was used by developers to test their games on actual hardware and best of all, these flash carts will even run on a retail Nintendo DS,
with that said though, my copy of "Cradle of Egypt" for some reason won't run on a Nintendo DSi (XL), or a Nintendo 3DS, however it ran flawlessly on my first generation Nintendo DS.
The flash cart was to be used with the IS-NITRO WRITER, a writer that would even write flash carts in bulks of 8, or copy the data from one "master" cartridge to 7 other cartridges.
** Picture above borrowed from Wikipedia
The IS-NITRO was the official NDS development, invented by Intelligent Systems who has quite a track record of making development kits for Nintendo's hardware.
As far as I know the cartridge only exists in a 512Mbit version, which most likely is the maximum size a DS game can be, however I don't think any DS game ever was 512mbits, but I could be wrong.
Underneath a small lid on the backside of the cart there's a slot for various types of saving, in this case the cart is equipped with a 4k EEPROM. If a chip for saving is nonexistent, a small switch is
available on the cartridge board to deactivate the feature.
I guess the changable save chip is the sole reason why the cartridge is as tall as it is.
This is it, thanks for reading :-)