markings on window balance

Hello, thank you for posting,

With this combination of attachments, you could have either an S380 or S385 channel balance. You’ll need to measure as shown below. The Series 380 has a 1" bottom attachment, and the Series 385 has a 1-3/32" attachment.

The length of the metal channel excluding the nylon attachments should measure 27" and you will need a 2640 stamp option. Please review and let us know if you have any additional questions. 

Comparison of the S380 and S385 balance shoes.