Window balance for a Millard window

A quick learner from Seattle, WA says:
I have several sliding windows throughout the house, and one of them has no channel balances (no clue where they went, missing since I bought the house 3 years ago). I am finally getting around to replacing them and one of the windows uses the balance as pictured in the attached image. Another balance has "25 3 GL Caldwell" as its markings instead. The pictured balance is in a window pretty close to the same size as the one missing its balances. Not sure how to figure out the right parts to buy.
User submitted a photo of a window balance.
Profile picture of Tom
Tom from SWISCO responded:
It's always tricky finding replacement balances. Fortunately, yours aren't that complicated, so we may be able to get something going for you.

The easiest part is the Series. If this window is the same type as the other two, then I think it's likely that it uses channel balances, as well. In these instances, I like to recommend our Series 385 channel balance. The nylon attachments have wings, which should stabilize it in the track, but if you find they get in the way you can always clip them off. The bottom shoe is also a little bigger than other Series, so if that works then great, but if not you can shave it down to size. It's better to do this than have a shoe that is too small that needs to be built up. Does that make sense?

Now, for length. I notice your two windows both have 26" long balances, according to their stamps. I'm hoping that would work for you in this other window, but you should absolutely double check just to make certain. Does the missing balance window have the same height as the two others? If so, 26" may work. If not, tell me the height of the visible glass on each sash and we can recommend something.

Once you have the Series and the length down, all that's left is choosing the correct stamp option. This indicates the strength of the spring, so all you have to do is weigh the sash on a scale. We have a chart on each Series 385 store page that shows each option and how much weight they can support. Once you have a length, you just have to compare your sash weight to the chart.

Hopefully this isn't too confusing. Let me know what the length would be, or if you need us to calculate it for you based on glass size.
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