Can someone please tell me what the function of E21/E23 was?

Also, I plan to put the drop gate down so I am assuming the part on the left in the bottom pic. The sheet only shows the one part and the gate looking like it is going in closed. I don't want to be "Dragoned" again by their instruction sheets. Be pretty bonehead to have the thing sticking up in the middle of the open gate.

Hi,
I think it is to lock the gun when travelling. One for the locked version and the alternative for unlocked when in action.
Greetings

I was looking at the one that's upright and wondering if that was the reason. It look like it could support the rear of the gun and t looks like there might be enough travel for it to reach back that far. But then why have a support for the barrel that can be raised and lowered from the fighting compartment?
(need a head scratching smiley)