I would refer you to ifixit’s website. They have complete tear downs of iPhones. If you search for the iPhone 5s/SE, refer to the teardown and they should have pictures that might identify that specific part and where it belongs.
https://www.ifixit.com/
Is there a spreadsheet with a coneolete parts list avaialbale? I think they publish them with the cost to manufacture reports:
Interesting problem. Searched high and low, but nowhere anything like this thing.
Its rubber, yes? What in particular is 3mm and 2mm - your measures do not fit anything. Put a ruler aside and take another picture ;-)
It reminds me of a small piece that was on the iPhone 5s, but it's been a couple years since I was doing iPhone repairs. If it is that part, it would be adhered on the display assembly next to the front-facing camera, and then a sensor is stuck down inside it. If your replacement display didn't include the small parts, you'll need to stick it onto the screen first before screwing the cables, speaker, cameras, etc. in.
Unfortunately, I can't find a picture that clearly shows the part I'm talking about.
That goes right next to the front camera.
It sits on an ambient light sensor which is where the little bent ribbon part of the front camera assembly is.
Right here marked on red:
http://prntscr.com/ipnfla
Look at the front camera cable and match it up to that place, it sits right over that thing like I said above and mounts right of the front camera plastic mount.
[doublepost=1562547079][/doublepost]I just spent an hour trying to find out where that stupid little piece goes, after fixing my daughter's iPhone> I LOVE YOU for posting this answer!I did a Google Image search with the photo you provided, and one of the search results is you asking the same question on iFixIt's website, to which I can see another user gave you the answer. Hoorah, problem solved!
I can't help but feel slightly creepy, but I promise all searches were done with the intentions of helping.
In case the OP disappears for good, here is @benjamen50's answer on iFixIt:
You're so welcome![doublepost=1562547079][/doublepost]I just spent an hour trying to find out where that stupid little piece goes, after fixing my daughter's iPhone> I LOVE YOU for posting this answer!