ID window sash thing

Hi there. It looks like you have either a Series 378, Series 380, or Series 385 Channel Balance. The only difference between these three balances is the 'depth' of the bottom nylon shoe, as shown below. Measure your old balance's shoe depth to see what matches best. See the video below that shows how to further ID the exact replacement.

Comparison of the S378, S380 and S385 balance shoes.