Window balance

Thanks for posting. If the balances stamped 1720 have 18" long metal channels, and the balances stamped 2420 are 25" long (not including their end attachments), then I believe you have either the S351-18 and S351-25, or the S390-18 and S351-18. The only difference between these two balance Series is the 'depth' of the bottom nylon shoe. See below. Measure your old balance's shoe depth to see what matches best. Let me know if you have further questions.

Comparison of the S351 and S390 balance shoes.