I am at a loss here
https://www.alldatasheet.com/datasheet-pdf/pdf/17821/PHILIPS/ISP1581BD.html is the datasheet for that chip, which is the only one on there (the silver thing on the back that is not a USB socket is likely a clock crystal). Everything else is just a passive.
Anyway that chip is just a USB controller. No particular brains or anything behind it you can program which makes it all the odder.
It does not resemble any homebrew type device I ever heard of for the GBA or for the DS -- normally they were things like serial out, rumble, extra memory or flash carts. I would like to believe I am fairly on top of homebrew goings on here (there is a reason
https://web.archive.org/web/20120119171132/http://www.reinerziegler.de/ and
http://www.brolinembedded.se/projects/keyboard/ and more are in my bookmarks). The PCB looks professionally made (though PCB fab costs during the DS lifetime dropped like a stone so you could do very small runs quite happily, and many universities would have options in house you could use) and hand soldered by someone rather generous (but good -- surface mount is not easy) with their solder. Nor does it look built down to a price as it has various component sizes on there which I would have not expected from a commercial operation operating a pick and place and having that much free space on the circuit.
The guesses then would be some kind of data out (debugger maybe), save dumper or controller input (or maybe output if someone wanted to control a PC with a DS and not use network options or drop back to a GBA and use a serial cable).
If I was less lazy I would match pins on the chip (already linked datasheet, you want page 4) to the GBA pins (
http://problemkaputt.de/gbatek.htm#auxgbagamepakbus ). The pins going from the chip to the right on the picture are AD pins address/data according to the sheet and the vias on the pins go to seemingly both the ROM read pins and the save (but also common enough communication protocol
http://problemkaputt.de/gbatek.htm#gbacartridges ) numbers.