Balance with stamp I am unable to identify

A quick learner from Vancouver WA says:
Hi there,

I have a window balance that is stamped 21 S B.S.I. I believe it coordinated with the 351 series. Can you please help me confirm?

Thank you,
User submitted image of their window hardware.
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Paul from SWISCO responded:
Hello, Amber. As long the bottom balance shoe has a depth of 1", then our S351-21 channel balance would be a good match. I haven't seen that kind of stamp in a long time. It's likely a very old balance. I believe 21S S might stand for a very strong spring, but I'm not sure. I recommend weighing your sash on a scale and referring to the chart we h ave on the S351-21 store page.
A quick learner from Vancouver Wa says:
Thanks for your help! Sometimes I have a hard time believing 1990 is a long time ago, but it is. My sash is 14 pounds, so it looks like the 2030 version is the one I need. The boot depth is an inch, so thank you for helping match up the right product.
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Paul from SWISCO responded:
I agree with everything stated, including the sense that 1990 was only yesterday. Oh well, that's how it goes. Let me know if we can be of any more help!
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