Window balance

These look like Series 351 or Series 390 channel balances. Measure the bottom shoes to see which of the two you need. Series 351 has 1" shoe, and Series 390 has a 1-1/8" shoe. If your metal bar is 29" for both balances, choose S351-29 or S390-29. If the length is different, choose a different length. Your stamp "option" may be 2840 to match "28 4" but you need to weigh your window sash to be sure. There is a chart in each balance's product description that tells you which option to choose, based on your window sash weight.