The Upholstery Forum

In one picture you see the wooden cushion back of an office chair with fives holes cut out. In the other picture you see the chair back with three inserts/prongs/clips that get inserted into the holes of the cushion back so that the cushion is mounted onto the chair. What are those three plastic inserts/prongs/clips called? They seem to be very worn and it's not seating into the holes properly for a flush fit.