Okay hold on.
First, Bolt 18 you completely remove, this closes the filter oil supply valve and drains the filter housing so you don't have a mess when you pull the filter cover.
Second, the valve in the picture is an anti drain back valve between the rear filter and the front one. It's located in the front filter housing i believe, hydraulics will function fine without it. My 5488 had the same valve loose when I split it, they are held by a cheesy washer with a spring, I put a tack weld on mine to hold it in permanently, I'll see if I can find some pics.