This is a connector for the rails of IKEA Vigda curtain track systems. Personally, I find the connectors that came with Vigda complicated and flimsy, especially when the hex bolts are already there.
The connector repurposes the hex bolts from the end terminators.
The STL file is only the gray piece. Place the connector halfway in the rail, and fasten with the bolt; it should be a snug fit.
Here you can download the connector STL file: connector-STL-file