are you sure it does attach in the center? would make since not too. the 2 on the sides are sufficient to hold it in place.
the rivets shown from the top show me that the ones near the neck line attach to the "ropes" as you call them on the lames on the neck. there is also a pin rivet that keeps the front and back closed.
if it does attach, the top lame does not move very much.
is teh upside down picture the same as this one?
http://www.myArmoury.com/talk/download.php?id=8788 because in this picture it looks like the top lame articulates off of the rivets. i know in some lame articulations teh articulating rivet is used as dual purpose: to articulate and to hold the leathers or roping for other moveing areas.
becareful with artist drawings. sometimes they can add ifs and butts where there is nothing there.