Replacement support balance for Alenco Windows

A handy person from Grass Valley, CA says:
I am a maintenance person at a school. I have two Alenco windows that I believe need new support balances. The two windows appear to be the same size. Each window had different support balances. One had two 2730's and one had one 2820 and was missing the other. I need help trying to figure out, based on the windows, what is the correct size window balance. I also need the clips that hold the balance in. Thanks
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Bob from SWISCO responded:

Thanks for writing in. We can help you replace these balances, but we'll need to see pictures of a balance from each window. Specifically, we need to see pictures of the nylon fittings on either end, as well as the stamp code on the metal channels. The length of the metal channels, not including the nylon fittings, would also be most helpful. From there, we'll try and narrow down what we need. Many thanks.

A handy person from Grass Valley, CA says:
Here are the photos. The metal portion of 2820 is 29 inches long and the metal portion of 2730 is 28 inches. Thanks
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Bob from SWISCO responded:

Thanks for the reply, this is exactly what we needed to see! Looks like you need either our S351-28 or S390-28 with a stamp option 2730, as well as either our S351-29 or S390-29 with a stamp option of 2820. The only difference between the Series 351 and Series 390 is the 'depth' of the bottom nylon shoe, so compare both Series to your old balances to see what matches best. 

A handy person from Grass Valley, CA says:
Thanks. These windows are the same size so it is unlikely that I would need both sizes. Can you help me figure out which size is appropriate for the windows that I have? or will either size fit? Or do you use both sizes in the same window. I will compare for the depth of the nylon shoe. Thanks
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Tom from SWISCO responded:

We don't think you need both sizes either. You need one of these two Series, we just don't know which one.

I'm afraid we can't really narrow it down any more from here, though. Like Bob said, the only difference between them is the size of the shoe. You'll have to measure yours as accurately as you can to determine which option is right for you. The difference is important, but subtle, so it's not something we can really eyeball through photographs. You'll have to use your best judgement.

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