option

The option number represent the strength of the balance. You should choose the option that matches the stamp number on the balances you are replacing.

Here are the strengths for the 24" Series 377 stamp options:
2310 will carry a window sash weight range of 5 through 9 pounds
2320 will carry a window sash weight range of 9 through 15 pounds
2330 will carry a window sash weight range of 15 through 18 pounds
2340 will carry a window sash weight range of 18 through 22 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.