I have two window sash balances I need to replace

Thanks for posting, and nice work. Based on your report, yes, you need the S385-13. The only question is that stamp option. We don't carry this balance in stock with a 1250 stamp; but if your window sash weighs exactly 11.2 pounds, then the 1240 stamp would work as you suggested, since it can support a sash in the weight range of 11 through 12 pounds. Did the window ever feel difficult to close or keep closed, prior to the balances failing? That would be a sign the original calibration was too strong (and evidence the 12-5, or 1250, stamp was incorrect). Either way, if the weight falls within the 1240 range, I think this will work fine for you, but double-check the weight to be safe, and let me know if you have further questions. 

You're very welcome! If your sash was only 11.2 pounds then the original 12-5 stamp was definitely too strong. That would absolutely explain why it was so hard to close. 1240 is a great pick, so you should be all set.

Please feel free to reach out if you had an other question. We're always here to help!