Spring balance
Hello there. It looks like you might have the S351-29 or maybe the S390-29 Channel Balance. The only difference between these two balances is the 'depth' of the bottom nylon shoe, as shown below. Measure your old shoe's balance to see what matches best. For a stamp option choose 2820.
A 'stamp option' designates the weight range that a balance can support. If you go to the S351-29 and S390-29 shop pages I'll sent you, you will see you cannot purchase a balance without first selecting a stamp option. Your old balance was stamped 2820. Therefore you would select 2820 for your replacement balances. This stamp option denotes a balance that can support a window sash in the weight range of 12 through 16 pounds.
I hope this clears everything up for you. Let us know if you need anything else.