18" Channel Balances with stamp 17 S

A home owner from Washington State says:
I am fixing my daughter's windows and need to repair some Channel Balances.

One I can not identify is a 1/2 inch 18 inches long, but has 17 S stamped on the back. The others I have been able to find on your site.

Can you point me at the correct one?

A customer submitted photo of channel balance attachments.
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Paul from SWISCO responded:
Thanks for posting! It looks like you have a Series 390, but the S in your stamp certainly is puzzling. If you remove your sash from the window and weigh it, though, I would be able to tell you the best stamp option.
A home owner from Washington says:
As far as I can tell the weight is 4.2 oz. A small part of the spring is broken off, so the real unit could be just a bit more.

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Paul from SWISCO responded:
I'm sorry, but we will need the weight of the sash, not the balance. Please refer to our Guide on Installing and Removing Channel Balances, the first half of which tells you how to remove the sash from your window. Please make sure to give us the weight in pounds.
A home owner from washington says:
The sash weight is 17.5 lbs. Hope that helps.
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Donna from SWISCO responded:
Thanks for the information. It appears that you will need a S390-18 with the option 1750.
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