S380-34 stamp question

A professional from Naperville, IL says:
This is the balance and top/bottom clip set i need. I only need 1 pair for one bottom sash on a non-tilt single hung. I weighed the sash and it was 40lbs, and there is no stamp on the old damaged balance. Do i need to get a certain stamp or just order normal?
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Tom from SWISCO responded:
Hello! You would need a custom balance. I'm thinking of the 3370 option, which can carry 39 through 42 pounds. Would that work? I'll send you a custom order link for two of these that you can add to your cart.
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