Old balances

Thanks for posting. At first glance this channel balance looks like a Series 770. Please measure the length of the metal channel to confirm the length needed; I suspect you'll need either the 30" or 29" version. For these balances you also have to choose a stamp option, which designates the balance's strength. Your stamp looks a little unusual from what I can see. To know what stamp option to pick, weigh the window sash that these balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under either the S770-29 or S770-30 shop page. 

You also might want to check if the pivot shoes further down the track that the clip of the balance attaches to are in need of replacement. Without seeing the hardware removed from the window I can't be certain what you need, but it might be the 15-129 shoe. Take a look, and also see the guides on this page for removal and replacement information. Let me know if you have further questions.