3090 BSI AL 35

Thanks for posting. This appears to be either an S351-31 or S390-31 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement; see the diagram below for reference. Compare both with your old balance to see what matches best.

Comparison of the S351 and S390 balance shoes.

You'll then have to choose a stamp option, which indicates the strength of the balance. Your old stamp of 3090 is from an obsolete rating system; if your window sash weighs 21.6 pounds, then choose stamp 3030, which will support a window sash in the weight range of 16 through 23 pounds. Let me know if you have further questions.