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.