Anderson Window Balance Identification

A quick learner from Manassas, VA says:
August 4, 2013
My double hung windows are 29 years old. Openning is approx 28"x61". Top sash is 31" tall, bottom is 31.5" tall.

Looking at the balances, the left one (looking on inside) has numbers F1 8 31 2 0 stamped on it, the right one has 11600-2 on it.

do you have replacements for these models ?
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Tom from SWISCO responded:
August 5, 2013
Check out our 24-837. This can replace balance boxes with the stamp 8 31.
Andersen Sash Balance, 823, 827, 831, 837
Andersen Sash Balance, 823, 827, 831, 837
24-837
$54.20
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