Required number for sash window balance

A quick learner says:
for item S390-30, requires a number (2910 - 2950) for adding into shopping basket. How do i know which number to click on which will reference to S390-39
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Casey from SWISCO responded:
The required number option is a number that can normally be found on the original balance, it indicates the strength. If you cannot locate the stamp number, please remove the window sash and place it on a scale. The weight of the window sash in pounds is needed to determine the proper stamp option.
A quick learner says:
Thank you Donna.
There are two sizes of window
The weight of Windows are:-
(a)larger window is 36 lbs.
(b)smaller window is 24 lbs.
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Casey from SWISCO responded:
You had mentioned S390-30 and S390-39.
I am assuming the 30" balance is for the sash that weighs 24 pounds and the 39" balance is for the sash that weighs 36 pounds, please correct me if I am wrong.

The S390-30 with stamp option 2940 will carry a window sash weight range of 23 through 28 pounds.

The S390-39 with stamp option 3850 will carry a window sash weight range of 30 through 36 pounds.

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.

I can email you a custom ordering link for the 39" balances, just need quantity.
A quick learner from Florida says:
sorry, my typing mistake.
Both window balances has the same length of 30".
I would need total 12 balances. 11 for windows weight 24 lbs. and 1 for window weight 36 lbs.
Should the order for the 24lbs sash be: S390-30 Stamp option 2940?
What will be the stamp option for the larger 36 lbs sash?, it has channel balance length of 30" also.
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Casey from SWISCO responded:
I can look into custom ordering the custom weight balance for you, 36 pounds is heavy for a 30" balance, the standard options will not work.
You asked for 1 but it is recommended to replace both balances at the same time, replacing both window balances at the same time prevents extra wear-and-tear to the new balance, which will have to work twice as hard when paired with an old balance. Do you still only want to order 1?

The S390-30 with stamp option 2940 will carry a window sash weight range of 23 through 28 pounds.

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
A quick learner says:
Please order 2 balances for the 36 lbs window .
I will need 12 balances S390-30
Please advice how to do this order, so we can have all shipped together.
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Casey from SWISCO responded:
I will email you a custom ordering link for (2) 30" Series balances for a 36 pound sash shortly. You will be able to add other needed hardware to your cart.
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