It seems that it was mail-order only, in Japan, and was used by only two games : Pokémon Channel, and Animal Crossing e+
Enough of writing, let's see the pics.
Here is the box :



Mine is a bit worn sadly.
The box had this label on its back, dunno what it means :

Once opened :

And this is the SD Adapter :









Once inserted, the SD is flush to the adapter :


Running swiss on my GameCube, the adapter seems to behave like any SD adapter.