Finally I recognize your sig pic Ian....Contra: Alien wars, great game, impossible but great. I've only beat it once with the help of an emulator (thank god for save states)
Anyways it looks like that plastic part attached to the smaller thicker loop is where your thumb goes, and the plastic is what you touch the screen with (there for no smudge and smaller point of impact). And as Paladin said, it looks like the thinner loop would attach to the DS somehow.