Milgard balance ID help

A contractor from California says:
I'm working on a Milgard vinyl, non-tilt-out, single-hung window, 10-15 years old, that needs balances. The problem is, the original balances are gone. The overall window dimensions are 44 3/8" x 64 1/2", the sash is 42 3/4" x 32", 1 5/8" thick, the channel is 5/8". It is glazed with a 1/8" over 1/8 IGU. We tried 2830's that were suggested by someone else, but they don't feel right and keep popping off. Is there a way to identify exactly what should be in there?
Thank you for any help you can give,
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Tom from SWISCO responded:
Without the original balances handy, it will be hard to know what Series you need. You see, the stamp you mentioned (2830) only signifies how long the metal channel is and how strong the springs are. Plus, you said that the 2830 didn't work well, so there's a good chance that's not even right. The matter of the nylon attachments, the combination of which makes up the Series, is still an unknown factor. There is no information on the window that could tell us what Series you need. The only way to know for sure is to compare with the old balance.

There is one solution you could consider, but I cannot guarantee that it will work. Our Series 385 is a popular combination of nylon attachments used by many window brands. You could see if this works as-is, but if it doesn't you can always try to alter it by shaving down the bottom shoe or clipping off the wings, as necessary. It's not the most elegant solution but it may be your best bet.

As for the appropriate length and spring strength, if you tell me the height of the visible glass (for the sash these balances support, that is) and the weight of the sash (determined by placing it on a scale), I can make a recommendation.
A contractor from California says:
Yes, I wish we had the originals. The daylight is 29", and the weight is about 30/32 lbs. The series 385 looks like a good possibility, I think I will go ahead and try them.
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Paul from SWISCO responded:
Great, thank you. I'm a little confused, though. Just to confirm, when you say "daylight," do you mean the size of the visible glass? If the height is 29", I would recommend our S385-29 balance with a 2850 stamp option. However, you mentioned that the 2830 balance you had before (also 29" long) kept popping off. Did it pop off because the size wasn't correct?
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