Many aid climbs, particularly in Yosemite, have sections aided on rivets (very short small bolts without hangers) that require a loop of something to utilise them. These Butterfly Rivet Hangers are designed to do just that and feature an offset sliding loop section that can be cinched up to hold the rivet and sits firmly once weight is applied to it.
They are a little more secure than standard Rivet Hangers, but a little less strong.