Andersen Perma-Shield Narroline double-hung window sash counter balance. Sold individually. Can be used to replace balance boxes with the following stamped numbers: R18/1337, R18/1327, and R18/1331
Good Idea!
Balances are sold individually, however, it is a good idea to replace both window balances at the same time. Pairing a new balance with an old one would cause the new balance to wear out faster.