Which stamp option should I choose?

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.