Not sure which one I need?

Thanks for posting! It looks like you have either the S380-28 with a 1" shoe or the S385-28 with a 1-3/32" shoe. The metal channel of these balances is 28". 

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 2740 stamp option.

Comparison of the S378, S380 and S385 balance shoes.