I don't have a photo, but my William Evans has a half circle doll's head similar to that in the first photo in this thread with the addition of a third bite in the doll's head. The doll's head in the photo above does not appear to have a bite.

I have seen the Greener Cross Bolt on some fairly inexpensive shotguns, but one has to ask about the fit. A third fastener does nothing if it is not well fitted.

I've seen what I think is a "kirsten" third fastener on a SxS double rifle, this is like two Greeners, one on each side as opposed to a single one in the center. More commonly seen on O/Us. As a side note, this rifle was chambered for the 9.3 X 74R and was so light that I would have been afraid of the recoil. Goes to show a double doesn't always weigh more than a bolt action rifle.