Caldwell FM 28 4

Hello,

We can't be positive with the balance still installed but it looks like you may have either our S351-29, with a 15-002 bottom attachment, or S390-29, with a 15-001 bottom attachment. With that being said, you would need to confirm that both top and bottom fittings do not have wings as shown in our photo. You'll also need to measure your shoe depth compared to the image below to determine which balance series will work for your window. You'll also need a 2840 spring strength option for your replacement balances. Take a look and let us know if you have any other questions. 

Comparison of the S351 and S390 balance shoes.