Three guesses.

[adam quantrill]

What's this then?

[jimburr]

[jimburr]

Is it a trophy like the one I tried to upload a picture of?😁

[adam quantrill]

No it's off the 400SX!

[mark387mw]

Chassis plate to outrigger?

[adam quantrill]

You're getting very warm!

[SEvans]

Some metal off the chassis diff carrier?

[mrzigazaga]

All that is left of an old shoe that you left in the boot when you bought the car?

[Wedg1e]

Going by the general proportions and the two bolt holes, it looks like it used to run along the back of the chassis holding up the boot floor (I'm guessing this is what Steve meant)...

[Wedg1e]

...have you fixed it yet Adam?