Andersen&reg; 531 Narroline Sash Balancer Works with Perma-Shield Narroline Double-Hung Windows manufactured from 1968 to Present. Sash balancers are metal boxes that contain a nylon cord under tension. The balancer number is a code for the tension and the cord length. The proper balancer must be used to maintain the lower and upper sash in an open position. If you replace one balancer you should replace both balancers.