Strap slack tensioner i call em... not sure what the tech word is...
These come in all sizes and metals... they were on shoulder straps and belts and various items where the slack adjustment is needed for sizing something in length,,, this one being in a park perhaps was from band instrument or uniform strap or shoulder bag strap etc... maybe even belt type item.... also on arms for slings... etc etc. even on gun straps.
These also have come out of swimming holes from old time bathing (male) suits.
ARC got it right... your find is a strap length-adjuster.
The precisely-measured length of the slot in your find will help us to narrow down the possibilities for its specific ID.
Also, please note that it has truly "circular" (round, not just smoothed-down) corners, instead of the usual 90-degree corners, which can cause snags, and scrape your skin. The circular corners are another important clue for determining the specific ID of your find.
For example, circular corners on a strap-adjuster strongly indicate it is from sometime in the 20th-Century.