Jack, I think you are correct about the hidden third fastener not helping much to keep the barrels on face during the firing sequence. I have always considered it as an assist or backup to the primary bolt.

A bolted top extension, however, does lock the barrels to the face. I believe a dolls head needs to have a bolt to keep it seated, otherwise there is enough flexing of the action bar that it will come out of it's seat during firing, and be rendered ineffective.

Any top extension when fitted properly does help stabilize the barrels laterally.

I am a great admirer of the Bissel rising bite as an effective top fastener, but it's hard to beat a Tolley giant grip for sheer coolness.