Need new spring balance

Thanks for posting. It looks like this is either an S380-18 or S385-18 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the diagram below for reference. Measure the shoe on your old balance to see what matches best. Let me know if neither seems right.

Comparison of the S380 and S385 balance shoes.

For the stamp option, if the window sash in fact weighs between 6 and 8 pounds, a stamp option of 1710 should work, as this designates a balance that can support a sash in the weight range of 5 through 8 pounds. But I strongly advise obtaining the exact weight of the sash to ensure it falls in that range. Otherwise it is not going to work. Let me know if you have further questions. 

Yes, you'll want the S380-18 then with a stamp option of 1710. That should work just fine. Let us know if you have any other questions! 

My pleasure! Let us know if you need anything else, and thanks again for reaching out.