21" channel

Hello, thank you for posting,

It appears you have either our Series 351, with a 15-002 bottom attachment, or Series 390, with a 15-001 bottom attachment. Take a look at the image below to measure and determine which balance you'll need. 

A 20B stamp should translate to a 2020 stamp option, but we recommend weighing the sash to confirm it falls into the 8 to 12 pound weight range. Please review and let us know if you have any additional questions! 

Comparison of the S351 and S390 balance shoes.