Pls help me put this mysterious rubber seal back in its place - 850, 1995

[geodonuts]

This round black rubber ring appeared somewhere in the process of removing the fuel tank, hopefully for the last time of this car’s life, and i cant seem to figure out how it fits, presumably over the hole for the fuel pump access inside the wagon trunk. I will add a picture in a moment, still figuring that part out I know it doesnt cover the inside side of the access port because Ive used the metal edges to lever a screwdriver against in order to unscrew the plastic lock rings for the sender unit etc, and there are bent notches to show this. Does it affix to the outside somehow, via the removable steel ring plate which holds the upward facing screws which the inside plate cover attaches to? How have I missed seeing this until now, when it falls out?

A mystery, and Id hate to figure this out after I get the tank reinstalled but better late than never. Pls share your thoughts if you have any, thank you always!

[geodonuts]

https://postimg.cc/CztY0jrZ

[Clemens]

late, but still: is there a seal around the fuel pump hole? I did a few fuel pumps a few years ago, and I'm quite sure there is a seal which would be that size.

Maybe that pic helps?:

[Clemens]

Forget what I wrote above. There seem to be no seals around the pump cover.
But maybe it is this #17 seal in the following picture:

[BlackBart]

What about #8?

[Clemens]

What about #8?

witchcraft, you just put that there ...
well, at least my memory is better than my latenight exploded view reading skills.

[BlackBart]

I just remembered a big rubber seal under the lid.