What stamp do I need for my S770-28?

A quick learner from Mint Hill, NC says:
April 29, 2015
I am returning the S775 balances that I purchased.
The style and/or model that was ordered was incorrect. The model/part style that should have been ordered was 770/28". I do however have a question regarding what the difference is between a "28C,28D,etc. The ones currently installed read 2800BSI.
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Tom from SWISCO responded:
April 29, 2015
Sorry to hear you're having trouble. The stamps are actually representative of the strength of the spring. There's actually a chart on the S770-28 store page that shows each option and how much weight they can support. All you need to do is weigh your sash and match that up to this chart to determine which is best.
28" Series 770 Balance
28" Series 770 Balance
S770-28
$12.70
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