How to determine stamp option if stamp has worn off?

A handy person from California says:
How do you determine a balance's stamped number if it has been worn away or the balance is missing altogether? I have four sliding windows in which three have no balances and the balances in the fourth have no four digit number as they have worn away over time. These windows were installed in 1962.
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Paul from SWISCO responded:
Thanks for posting! We can help you determine what stamp option you need, we just need a bit of information.

First, what is the length of your metal channel? Please do not include the plastic attachments in your measurement.

Second, what is the Series of your balance, as determined on our own site?

Finally, we will need to know the weight of your sash. You can remove it and place it on any common bathroom scale to determine this.
A handy person from California says:
According to your site the series I need is an S351-17 as the metal channel is 17" long. The sash weighs 10 pounds.
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Donna from SWISCO responded:
Choose stamp option 1620, this will carry a window sash weight range of 8 through and including 12 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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