What is this plastic piece that fell out of my iPhone4?

I'm replacing the midframe from my iPhone 4 and a small plastic piece with a screwhole came out from near the sensor array.

I'm not sure how to put it back because there's no mid-frame guide for the iPhone 4 and I don't see it in any of the sensor or power button guides.

Photos below. Does anyone have an image of this piece laying in the correct position? It just sort of jumped out after I undid the sensor cables (I think).

Images of both sides of the piece below:

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