Window channel balance

A handy person from Eastvale, CA says:
Hello, I need replacement for a 28" window channel balance. Stamped marking is BSI 27400 or the last letter might be a Q, 2740Q. Thx
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Bob from SWISCO responded:

Thanks for posting. This appears to be either an S351-28 or S390-28. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the diagram below for reference. In either case for a new stamp option choose 2740.

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.

A handy person from Eastvale, CA says:
Hi Bob, which one should I order if it's 1 1/16? Thank you.
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Bob from SWISCO responded:

If your shoe depth measures 1-1/16", then odds are it started out as 1-1/8" when it was brand new. These are only made of plastic so friction from constant use tends to wear them down over time. With that in mind, my recommendation would be the Series 390.

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