What IS this?

grimreaper said

I hope these photos help. The back of the rear seat would go against it and carpet covers it. The divider board goes against the back of it.
View attachment 1715296828 View attachment 1715296829

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Here's my trunk view (not quite as nice as yours, but I'll try to get close). My existing original piece (in this picture) is much shorter than the black 'mystery' piece, but the width , the spacing of the holes, the general design & the notch for the gas filler are identical.