This is a claviature for the C64, connected through the cartridge port. It is mapped to $df00-$df06, and every key corresponds to one bit each in those seven bytes respectively, except for $df06, where only bit 0 is used. This is because the keyboard consists of (6*8)+1 keys, and therefore 6 bytes is not enough to cover all of the keys. A bit weird, but that is how we like it.
Picture borrowed from here.