21'' by 1/2'' balancer

Hello,

It looks like you have either our S351-21, with a 15-002 bottom attachment, or S390-21, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need a 2030 spring strength option for your balances. 

Choose the spring strength option based on the stamp on your balance.

Comparison of the S351 and S390 balance shoes.