Window Balance

This appears to be either an S351-29 or S390-29. The only difference between these two channel balances is the depth of the bottom nylon shoe, so compare both with your old hardware to see what matches best. For a new stamp option, I believe you might need 2820, which means the balance will support a window sash in the weight range of 12 through 16 pounds. But you might want to obtain the exact weight of the sash, perhaps via a bathroom scale, to confirm. 

Comparison of the S351 and S390 balance shoes.