Window Sash balance

It appears you have either the S380-22 with a 1" shoe or the S385-22 with a 1-3/32" shoe. To determine which series is needed for your window. Look over the dimensions from the diagram below to confirm the depth of your bottom shoe. Let me know if it does not match either measurement. 

You'll need to select the 2140 stamp for a window weighing 15 to 23lbs. Thanks for writing!

Comparison of the S378, S380 and S385 balance shoes.

So, you'll need the S385-22 channel balance. It is likely the shoe is slightly worn down due to the friction over the years, opening and closing the window.