Need to ID Certainteed balance spring

Hey Ian. This looks like a 15-192 Certainteed Pivot Shoe, which uses Series 170 and Series 171 Coil Balances. For a 12 pound window, use the S170-406 6 pound spring, one on each side of the sash. This equals 12 pounds of support overall. You can reuse your old coil covers if they're undamaged--otherwise, see our 15-156 and 15-157. Let me know if you have further questions.