Andersen Narroline double-hung window sash counter balance. Sold individually. Can be used to replace balance boxes with stamped numbers R1445/27. See SWISCO product code 24-L1427-45 for the left handed balance for this window.
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.