28-9 stamped Sash balance

A home owner says:
I am looking for replacement sash balances and I identified the 29" 380 series as the one I need. However I don't know how to pick the stamp option. I don't see any match with what I see on my current balance (see attached). Can you help me pick the right one?

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Tom from SWISCO responded:
Hey, Vincent. Stamps that end in "9" or "90" use an old rating system that has fallen out of uses. Our standard options may still work for you, but you would need to weigh your sash to confirm. We have a chart on the S380-29 store page that lists how much weight each option can support.
A home owner says:
Thanks Tom. So to confirm, I will need to take off the lower sash (if I understand correctly this is the movable part of my no-tilt window) and weigh it?
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Paul from SWISCO responded:
Yes, the sash is the glass panel that slides up and down. It's what the balances support.
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