balance question

It is good that you identified the balance series and measured the length, the stamp option refers to the strength of the balance. Please remove the window sash and place it on a scale.
Stamp options for 23" Series 751:
22R will carry a window sash weight range of 10 through 18 pounds
22HD will carry a window sash weight range of 18 through 22 pounds
22TG will carry a window sash weight range of 22 through 30 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.