Sorry I can't help you as much as I'd like.
Here are some photos of the relevant parts of a 3d model double action (skull crusher). The gate is externally similar to the single action gates, but the backside is different. You'll also see the additional tiny screw above the thumb screw. Unfortunately, the head of my tiny screw has split in half, so I am currently unable to remove it.
From what I can see, on the double action, the thumb screw threads into a small flat spring located in a groove on the underside of the loading gate. That spring applies pressure to the frame to retard the gate movement. The small screw extends through the gate at the opposite end of the flat spring and protrudes into the groove in the frame. It doesn't seem to touch the end of the flat spring or bottom out in the groove, but in a quick look I can't see what it does.
I know this won't give you all you need, but perhaps it will help at least a little.
--DJ