35" channel balance, stamped with 34 3 c BSI

It looks like you probably have wings on both top and bottom attachments but you'll need to check the bottom since it's still installed. With this combination of attachments, you could have either an S380-35 or S385-35 channel balance. You need to measure as shown below. The Series 380 has a 1" bottom attachment, and the Series 385 has a 1-3/32" attachment. You'll also need a 3430 spring strength option for your balances. 

Thank you and let us know if you have any other questions or concerns.

Comparison of the S378, S380 and S385 balance shoes.