Window Balance Identification

A quick learner from Port Charlotte , FL says:
These photos are 2 balancers that are from the same set (6) windows ( same size and weight) they may be from 2 different manufacturers. They are 9/16 by 30". Please let me know what counter balance you recommend. I did weigh one of the windows and it weighes 41.4 lbs.
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Thumbnail of the uploaded file named 29DS37
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Tom from SWISCO responded:

Hello! These balances are definitely from two manufacturers, and both have a specially-calibrated spring. These may have to be custom made. Please give us a little more time to look into this and I'll get back to you ASAP!

In the meantime, can you refer to the diagram below and tell me which size is closest to the fittings on your balance?

Comparison of the S380 and S385 balance shoes.
Comparison of the S380 and S385 balance shoes.
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Tom from SWISCO responded:

Hello! I just wanted to let you know that we're all set on our end. We can supply this balance as a standard product, so no custom order is necessary! 

Basically, your old stamp (29DS37) indicates that your sash weighs 37 pounds. That means you can use our 2965 stamp option as a replacement, which is rated to carry 35 through 39 pounds. This is a normal stamp option for both the S380-30 and S385-30, so all you need to do at this point is determine which of these Series is the better choice.

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