Need28 Brand: Window Repair Systems 4.7 4.7 out of 5 stars 30 24" Caldwell Series 300 5/8" Channel Block & Tackle Window Balance -

Thanks for writing in! Your channel balance can be replaced with either the S351-24 or the S390-24. To confirm which of these is the best replacement match, you'll need confirm the measurement for the depth of your bottom shoe. 

To select the stamp from the list of options, you'll need to look for any stamps/markings on the metal channel.

Comparison of the S351 and S390 balance shoes.