Please help me replace my e14 8#31 881 window sash

A quick learner from Southampton ny says:
Hello, I would like to replace a window and the sashes have the number e14 8#31 881. What balancer would I use for that?
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Lauren from SWISCO responded:

Hello, thank you for posting,

For a balance box marked with 8#31, please review the 24-837 Andersen sash balance as a replacement. Please note that we cannot assist with replacement sashes. Let us know if you have any additional questions! 

A quick learner from Southampton ny says:
Hi, Lauren,
I have more questions.
I successfully replaced Andersen balances in two of my windows.
When I went to do my third window the strings were too short and the bottom sash wouldn't close all the way.
The box had the markings BP2 12L 6 8. The other side is the same, except it shows 12R.
Your message boards said that the 24 13L and 13R balances would work for that window.
My window is 73x36.
I ordered four boxes for two windows that I have in these two dimensions, two are not used (one L and one R) and one is partially installed in the window, I disconnected the front cords so I can close the window in the meantime.
What box would you recommend for this window? Can I return any of these boxes?
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Paul from SWISCO responded:

Hello, 

You ordered the correct balance boxes for your cottage windows, 24-13L & 24-13R. Anderson window company determines the handing from the outside looking in for directional left and right of the window. Being that the cord is too short for the bottom sash, switch the boxes around, put the 21-13L to your right and 24-13R to your left.

Let me know if this works out for you! 

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