You are correct that is the vent and yes that sprung washer is suppose to be like that, as its sprung to hold the vent in and allow it to open and close.
From what you have said about the level of ATF in the box, I would be draining some off or drain the lot, change the filter and fill with new to the correct level, whom ever serviced your car and put to much fluid in there in the first place doesnt seem to me to know what he is doing.....
VELVET BLUE UK INDIVIDUAL , IM THE LAST
UNITED WE STAND DIVIDED WE FALL
I stepped in a plate of Pasta the other day - now I have to worry about my Carbonara footprint!