Hi guys...yea the inner arms are attached to the screw drive and they push down on the 1/4 frame and force the Draft arms To sissor,thus the box goes up....you are correct that you would not see those on a 1:1 1/4 frame end dump.
Poor pic but you can see where the inner arms attach to the screw drive.
I have pulled all 3 styles of end dumps....Full frame,frameless, and 1/4 frame.
Each have their good and bad...personaly I like the 1/4 frames as they are more stable than a frameless yet you can " inch worm" out if you get hung up after the load comes out....
First trailer I pulled was a full frame...nice and stable just don't get stuck!
I have not made a video but will try to make one this weekend...
Trent