Purple Heart Issue: What Is This?

I'm in the process of sorting out the Purple Heart Issues and putting them all on an album page.
I regularly come across stamps from different periods with an impression in the shape of what looks like a paperclip. See photo.
Often used on piece, but in the kiloware I also find sometimes single unused stamps with backing.

Who know what this is or how it came.

The most likely explanation: some non-profit veterans organizations will include a stamp paper-clipped to the enclosed materials soliciting donations to their organizations, in the hope that it might encourage the recipient to use it to send back a donation. These are almost always patriotic definitives, like flag stamps, and the purple heart varieties. The paper clip often leaves an impression on the stamp, such as you've demonstrated.

Agree, the shape of the paperclip is very visible on that central stamp.

Occam's Razor, if ever. Don't overthink it, it was a paper clip.

Thank you for your answer. I think it's a very plausible explanation. Has anyone received this kind of mail from veterans organizations for example to confirm this? If so, which one was that?

No need to see what other organizations do this. I've seen it multiple times, and the stamps are pretty much ruined. Done it myself, by accident.

I concur. I can attest firsthand that I have seen many stamps paper-clipped to solicitation letters. The purple heart stamps seem to be the most likely stamp design utilized for this purpose, at least on the on solicitations I have seen.