need new parts and not sure what to order

A home owner from Florida says:
We need some new balancer parts. I believe these are called "Block and Tackle". They measure 29" longt(he metal portion) and are 1/2" in width. Can you please let me know if you carry a replacement part? We need 10 of these.
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Casey from SWISCO responded:


With this combination of attachments, you could have either an S380-29 or S385-29 channel balance. You’ll need to measure as shown below. The Series 380 has a 1" bottom attachment, and the Series 385 has a 1-3/32" attachment. You'll also need 2820 spring strength options for your replacement balances.

Thank you and let us know if you have any other questions or concerns.

A home owner from Florida says:
Thanks so much for your response. I measured and will need the S380 with the 15-005. Can you elaborate on the 2820 spring strength options? I'm not sure what this is or how to find out what I might need.
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Casey from SWISCO responded:

Sure, the 28B stamp shown in your photo correlates to a 2820 strength option shown on the S380-29 product page, which means that the balance spring will support a window sash between 12 and 16 lbs. 

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