Thank you for reaching out to us. We'd be happy to help but we would need to see photos of the top and bottom fittings for this balance to determine what Series you'll need. Can you also weigh your sash on a scale and tell us its precise weight? That will allow us to confirm the appropriate spring strength.
Great, thank you! It appears you have either our S351-28 or S390-28 balance but the best option ultimately depends on the depth of the bottom shoe as shown in the image below. You'll need to measure the depth of yours to see which one matches up best.
Your original 2750 stamp is calibrated for a sash weighing 26 to 30 pounds which wouldn't be strong enough to support your 35-pound sash. You may want to go with the 2765 stamp instead for your replacements. A pair of balances with this stamp will support a sash weighing 33 to 36 pounds.
