Non tilt channel balance

A handy person from Miramar, Florida says:
I have determined that the channel balance I need is a 27" series 380, however, I am not sure which stamp option I need, as there is nothing visible on them. They are the kinco series mark 40/50 HP. H-LC50-53x77. Thanks for your help.
User submitted photos of a window balance.
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Tom from SWISCO responded:
If your balances have no stamp on them, you will need to weigh your sash and refer to the chart on our S380-27 store page. That is the only other way to determine what option will work.
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