Not sure what stamp to order for 18" series 770 balance

A handy person from wi says:
June 25, 2016

The stamp on my channel is 17S24. The channel length is 18".There are B.S.I. in small letters stamped perpendicular to the 17S24 and then in even smaller letters there is oc stamped running the same direction as the 17S24.Not sure what to order. Thanks.
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Tom from SWISCO responded:
June 27, 2016
I'm not entirely sure, either. I'm not familiar with that stamp. Could you tell me the weight of your sash?
18" Series 770 Balance
18" Series 770 Balance
S770-18
$11.73
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