Andersen Perma-Shield Narroline double-hung window sash counter balance. Can be used to replace balance boxes with the following stamped numbers: 13L and 12SL. Sold individually, see 24-13R for the opposite handed version. Replacing both window balances prevents extra wear-and-tear to the new balance, which will have to work twice as hard when paired with an old balance. For older Andersen balance units you may need to cut a notch in the head jamb with a chisel to allow for the tab of the new replacement balance to fit into the header.