Need to know the part to buy

Hello,

It looks like you have either our S351-25, with a 15-002 bottom attachment, or S390-25, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. 

If there is no stamp on your metal channel, you'll need to weigh your sash to determine which strength option you'll need. There is a chart on the product page that explains how to choose your option based on the sash weight. 

Comparison of the S351 and S390 balance shoes.