Replacing window balance

A handy person from N. Carolina says:
I need to replace balances in my friends home. The marking on the balance indicates a 3500 BSI There is also an XL after the BSI. I will be looking to replace 16 balances
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Tom from SWISCO responded:
That is an odd stamp, so I'm not sure what you have. Please remove the balance from the window and take a few pictures of it for us to see. I will specifically need to see pictures of the top and bottom. Thanks!
A handy person from North Carolina says:
Here are pictures that best represent the item I have.

I also have a question about moving the screw pilot hole in the vinyl casing. The current balance is 36" long. Some of the screw holes have been stripped out, so, should I replace the existing balance with 35" ones and move the hole down by 1 ""
User submitted a photo of a window balance.
User submitted photos of a window balance.
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Paul from SWISCO responded:
Thanks for the additional information! You have been very helpful. This looks like our Series 770 channel balance. As for moving it down, it may not be necessary. See our 62-200 screw plug, which you could install right in the jamb. Could that work for you?
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