Thought I would provide an update on my most recent learnings on this subject.
I found a picture of a damaged keyboard, revealing some of the inner workings of the key mechanism, and attached a cropped copy here. This shows a pair of side-by-side keys operating the back valves. At the A location appears a hinge point, and at B are small holes for accepting/supporting springs. From this, I figure the mechanism is like in the attached sketch, where the top lever acts on the second lever that operates the valve. To keep the top key levers from having slop in the rest position, I figure there must be a flat spot at the contact location when in the closed position, as in the picture, that keeps it fixed. That requires a close synchronization in the timing of the closing positions at the valve and the key levers, which must make it a bit finicky to set up. Plus, I can see how maintenance would be issue too, for example trying to replace a broken spring.
Not sure if anyone else out there has comments to this, but I suppose this is the answer, or an answer, to my initial question.
Don