You put one split rivet in the crotch, and two snaps on the butt piece, and an elastic strap with a female snap on it, from the split rivet. This strap can then be snapped onto one of the two snaps on the butt piece, both allowing for some adjustment, and also keeping the butt piece in place by being held together with to the crotch.
It means that the grey paint is kept within the natural boundaries of the frown. Compare an ANH frown with a ROTJ and I think you'll see what I mean.
ROTJ