This is a photo of the balance I need

Can you show us the nylon fitting on the other end of the balance? From there, we'll do our best to narrow this down for you. 

Thanks for getting back to me. If the metal channel, not including the nylon fittings, is 29" long, then you'll need either an S351-29 or S390-29. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware and see what matches best. You'll then need to choose a stamp option, which designates the weight range that the balance is meant to support. The balance in your photos has a stamp option of 2840, which is for a window sash in the weight range of 23 through 28 lbs. If this is the original balance from the windows, then you should be good to go. Let me know if you have any other questions. 

Do you mean the old one, not including the nylon fittings on either end, is 30" long? If so, does it have a different stamp code on the metal channel (it should). If there's no stamp code, we'll need the exact weight of the window sash. Please confirm it has the same nylon fittings as the balance you show in your photos. If not, we'll need to see the fititngs as well. 

Thanks for clarifying. Yes, if the metal part is 29" and the shoe depth is 1", then the S351-29 seems right to me.