Which stamp option should I choose?

A home owner from Phila PA says:
I have a 28 inch series 390 balance, I cannot see a stamp number on the channel, which should I choose?
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Donna from SWISCO responded:
Remove the window sash and place it on a scale.

These stamp options will carry a window sash weight range of the following given weights
2710: 5 through 9 pounds
2720: 9 through 13 pounds
2730: 13 through 19 pounds
2740: 19 through 26 pounds
2760: 30 through 33 pounds

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
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