Chris Cunningham wrote an article in Sea Kayaker (spring '93, I think) about building a S&G Greenland kayak. It had flush hatches with bolt-operated toggles that sounds very similar to the description of the system in this thread. As I recall, it had a knob on top attached to a bolt, which went through a wood toggle and into a Nyloc nut. There were blocks fastened to the underside of the cover to limit rotation to about 60 degrees, and the Nyloc caused the toggle to rotate once the tension was released.

Sea Kayaker has a web site with an index of past issues, and you can order directly from them, or NWOC has them as well. I have one in the stack at the back of the racks if you come up dry. Later.