need help identify parts windows balancer

Hello there. It looks like you have either the S351-27 or S390-27. The only difference between these two channel balances is the depth of the bottom nylon shoe, or what can be considered the 'heel-to-toe' measurement. Compare both with your old hardware to see what matches best. In either case, for a stamp option choose 2630.

Comparison of the S351 and S390 balance shoes.

Good question. The answer is no; stamp options are calibrated to support a very specific weight range. You don't want to use a balance meant for a higher weight range, because this will result in the window being very difficult to close or keep closed (since, relative to the sash weight, the balances are so strong that they'd keep 'pulling' the window upwards!). You want a balance with a weight range that your sash's weight falls precisely into. This is optimal. 

Now, your old balance had a stamp of 26-C. This translates to our stamp option of 2630. This will support a window sash in the weight range of 13 through 19 pounds. If you want to be extra-safe, you could always weigh the sash to ensure it falls in that range. Hope that all makes sense, and let me know if you have further questions! 

Perfect! That's great news. Please feel free to reach out if you needed anything else.