Difference between 3600 and 3601 balances

A home owner from Charlottesville, VA says:
What is the practical difference between S775-36 balances stamped 3600 and 3601? I seem to have both types in identical windows in my house. Is it possible that either could work for my particular window? Perhaps the sash weight suits both balances, for example.
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Donna from SWISCO responded:
The 3600 has a different weight tolerance than the 3601 option. It is best to replace to the original stamp and use the intended strength balance.

3600 will carry a window sash weight range of 9 through 24 pounds
3601 will carry a window sash weight range of 25 through 32 pounds

Sash weight ranges do not include the manufacturer's sash friction from use of weatherstripping. This friction must be verified by the manufacturer since all windows vary in sash friction due to manufacturing methods and weatherstrip types.
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