replacement balances for a Rivco window

A quick learner from Merrimack, NH says:
I need to replace a pair of tilt in double hung vinyl window balances.
They come out of a Rivco window.
The broken balace is marked with (11/2) Unique 30-C-30
The (11/2) is a guess as it is not completely legible.
Your series 770, 30" looks correct however I'm not sure of the "option"
30d, 30c, or 30b.
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Donna from SWISCO responded:
If you are sure you have the Series 770 channel balance you would need the S770-30 with the option 30C.

If you want to double check that you have the right replacement balance series picked out... send us photos of both the top and bottom of your existing balance. We will take a look at your pictures and try to confirm which balance series you need.
A quick learner from Toronto says:
sooo what do the different options indicate?
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Donna from SWISCO responded:
Thanks for posting! The stamp located on your channel balance indicates its strength; or, how much weight it can support. Keep in mind, however, that the stamp options for each balance differ depending on what Series balance you have, as well as its length.

For the S770-30, a 30B will carry a window sash weight range of 13 through and including 19 pounds; an option of 30C will carry 19 through and including 24 pounds; and finally 30D will carry 24 through and including 29 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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