window balance and parts replacement

A contractor from Berkeley says:
I need to know what a 24 B is equivalent to? I have 10 lb. sashes.
Thank you,
P
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Emily from SWISCO responded:

Hello!

It appears you have our S351-25  or S390-25 channel balance. The important difference between these two parts is the bottom shoe size. Our Series 351  comes attached with a 1" deep shoe, and the Series 390 comes attached with a 1-1/8" deep shoe. Measure as shown in the image below to determine the best match. You will also need the 2420 strength option for a sash weighing 9 to 15 pounds. 

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
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