replacement channel balances

A handy person from Berkeley California says:
I need to replace several channel balances whose springs have broken. They are for non-tilt aluminum windows that are old (from the 60's at least). They are the 1/2" wide type (actually a bit over 9/16") and the metal piece of the part is 22" long. There is a stamp on the part that says "21 B", and also "B.S.I." or "B.S.1."

It looks like several of your styles look similar, especially trying to tell the difference between the plastic guides on the ends. Can you provide some guidance?

User submitted photos of a window balance.
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Tom from SWISCO responded:
Hey, thanks for posting. I've narrowed this down to two possible channel balance Series: either our Series 351 or our Series 390 balances.

Now, as you can see, there is only one difference between these two Series, and that is their bottom shoe. The Series 351 uses the 15-002 while the Series 390 uses the 15-001. I can't quite tell what you have based on your pictures, though. Please carefully compare their dimensions to your own hardware and see which is a better match.
Comparison of <a href='Channel-Balance-Shoe/pd/Replacement-Sash-Support-Channel-Balance-Shoes/15-001'>15-001</a> & 15-002
A quick learner says:
Thanks, the bottom shoe on my balance looks a little different, but the dimension is 1 1/8". Thanks. I'll go online to order.
A quick learner says:
OK, thanks. Now that I know I need the series 390 (the shoe is 1 1/8"), how do I choose the stamp option/sash weight? The channel balance I'm replacing has a stamp that says "21 B" and also "B.S.I." or "B.S.1." Is that the sash weight? The window itself (including the frame around the glass) is approx 34" wide and 23 1/2" tall, if that helps.

I need channel balances for a smaller bathroom window, too, the metal part of the CB is 16" long. So I need to know how to choose the stamp option/sash weight for those too.

Profile picture of Tom
Tom from SWISCO responded:
No, 21 B is the stamp you want to pay attention to. Usually, that corresponds with our 2120 option, but you should weigh the sash on a scale just to be sure. We have a chart under each balance that lists every option available, as well as how much weight each option can support.
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