Balance Replacements

A quick learner from Charlotte, NC says:
I have several balance replacements I'd like to order, but I don't have the balance for some windows. The one I do have is a 29 inch and it has 2900 BSI LD stamped on it. What does this tell me with regard to the right fit? How can I determine the others fit? Lastly, the bottom piece which I don't know the name of will turn and move, but there is no opening at the top to remove them. Is there some trick to that? I may see if I can make those work for now, they seem like they are ok. It's just the springs that are broken on many of our windows. Thank you.
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Tom from SWISCO responded:
I'm afraid I cannot identify what balance you have based only on that stamp. I would need to see pictures of it removed from the window so that I can match it up to what we carry. As for the windows missing balances, that's an entirely different story. If the windows with missing balances are identical to the windows that still have balances, then we can surmise that they use the same hardware. If they are different, however, then it will make matters a little more complicated.

As for the bottom piece you mention, it sounds like a pivot shoe. See the two videos below.
A quick learner says:
Here are the pictures. They are all the same just different sizes. Does this help?
User submitted photos of a window balance.
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Paul from SWISCO responded:
Thank you. Could our S770-29 be a match? I cannot see the end of the clip to confirm if this is correct but see what you think. In order to identify the proper option, I recommend weighing your sash on a scale.
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