Thanks for the replies.
I did search for examples of the third fastener, and saw the hallowell page. One uses a similar system to mine but the second one seems to have a cam actuated top lever system.
So far this is what I have come up with(please excuse my lack of artistic ability)
http://i277.photobucket.com/albums/kk74/HorseoftheSea/Mobile%20Uploads/image.jpg