Window Sash Balance failed parts

Hello there. Those stamp codes only refer to the strength of the balances, not their identity. Based on the end fittings you have either an S351-26 or S390-26. The only difference between these two balances is the depth of the bottom nylon shoe, or what can be thought of as the 'heel-to-toe' measurement. See the illustration below to help figure out which one you need. 

Now, you'll also have to choose a stamp option. You mention you have balances stamped both 2520 and 2530, respectively. Were these balances from different windows? Please confirm before proceeding. If so, you'd want to choose the stamp option 2520 for however many balances you need with that stamp, and the same with the balances stamped 2530. 

Good question. For a sash that weighs exactly 14.7 pounds, stick with the 2520 stamp option, which supports a window sash in the weight range of 9 through 15 pounds. You want the weight of the sash to fall exactly within the weight range of the stamp option you choose; even if that weight is towards either end of the range, that is still more appropriate than choosing a weaker or stronger stamp option. If the sash weight falls even slightly outside of the weight range, the balances will not function correctly: they'll either make it very difficult to open the window, if the stamp option was meant for a lighter window; or, it will be very difficult to keep the window closed, if the stamp option was meant for a heavier sash. Hope this helps.