I Don't Remember...

[SpeedThrills]

...what this is.
I took my 74 Dart Sport apart many moons ago. I'm reassembling it as a 71 Duster. I'm pretty sure this is from inside the fender, but I don't know. I also don't know if it's needed on a 71 Duster.
There are two of them, the other one is opposite, for the other fender, I suppose.

Here's the same piece, flipped over.

 

[Bad Sport]

I believe those are the end pieces for the bumper filler.

Maybe?

 

[d55dave]

Yes, read bumper corner filler I believe. It goes between the bumper and the body.

 

[H22A4]

As others have said, it’s the rubber/plastic bumper filler for the rear bumper. You don’t need it if you also changed your rear bumper to a ‘71 Duster.

 

[SpeedThrills]

Ah, rear bumper gizmo's. Makes sense. Thanks, folks!