Clips and Fasteners for Exterior Single Piece Storm Windows

Hello,

We bought an old house and have discovered that the majority of the exterior storm windows are held in with paint, hope and a plastic clip or two. The rest of the clips are long gone. We were going to repair it, but the clips are not held in by screws and we can't figure out what the "holder" is and how to remove it so we can add new clips. The "not-a-screw" is metal and doesn't seem to twist using a pliers (we didn't want to force it).

Any insight would be massively appreciated!

Thanks!

Annie

Dave-- THANKS! One thing: how do we get the old ones off? We can't figure out a way to attach replacement plastic clips without taking the old rivets off.