Replacing Block and Tackle Channel Balance; hoping to verify correct replacement parts

Thanks for posting. I agree that this is likely an S380-15. The only other possibility is the S385-15. These two channel balances are nearly identical; the only difference is the 'depth' of the bottom nylon shoe, or the measurement from heel to toe. See the diagram below for reference. Measure the depth of your old balance's shoe to confirm which of these you need.

Comparison of the S380 and S385 balance shoes.

As far as the stamp option goes, your balances' old stamp of 14-2 correlates with our stamp option of 1420, which will support a window sash in the weight range of 6 through 10 pounds. If your window sashes indeed weigh 11.5 pounds, then this certainly falls outside that range. Have these windows always been difficult to keep open, i.e., this was not a recent failure of the balances? If so, and you are confident in the measured sash weight, then I think it's possible these windows were installed with incorrectly-calibrated balances, and I agree moving up to 1430 makes sense. This option will support a sash in the weight range of 10 through 12 pounds. Let me know if you have further questions. 

Yep, that sounds like the original balances were too weak for the sash weight. Either the installers miscalculated or they just installed whatever weight they happened to have on them at the time; such things unfortunately are known to happen.

Weighing your sashes will definitely verify exactly what stamps you need. Please feel free to reach out if you had any follow up questions, though. We're always here to assist!