Can't find a stamp on a window balance

A landlord from Sausalito, CA says:

Hello,

My name is Kaveh and I work for a property management company in California.

I need to order two replacement window balances. I am quite certain that I want the product S745-32 (32 1/4" Peachtree Balance), but I can not locate a stamp anywhere on the broken part. The window that they will be used on is a double-pane window that is mid-size (approx 36 inches by 44 inches). Do you have a suggestion as to what weight range I should purchase for? I have attached pictures of the broken balance, but I don't know if that will help you much. Please let me know. Thanks!

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Thumbnail of the uploaded file named IMG_6900.jpg
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Bob from SWISCO responded:

Hey Kaveh. I'm afraid that the dimensions of the window sash can't accurately tell us the weight. The only way to be sure what stamp option you need is to weigh the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the S745-32 product page. 

A landlord from Sausalito, CA says:
Thank you for your assistance.

Can I buy a heavier-weighted channel to be safe, or does the weight of the sash need to match the channel capacity exactly?
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Bob from SWISCO responded:

The exact weight of the sash must fall in the properly calibrated weight range. A stamp option too strong will prevent you from keeping the window closed; a stamp option too will prevent you from keeping the window open. You want the properly calibrated balances for the window sash's exact weight. 

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