Identify window sash balance part

Hey Yuchun. This looks like either an S380-27 or S385-27 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. Based on your photos you might have the Series 385; but photos of measuring tape can be deceiving, so be sure to double and triple-check your measurements to see which balance matches best. In either case, for a new stamp option choose 2650.

Comparison of the S380 and S385 balance shoes.

So long as it's deeper than 1", it's safe to assume it's the Series 385. Sometimes these shoes can wear down a bit from use--but the fact it's still greater than 1" is good enough to go off of. Let me know if you have further questions.