Marked "28C", 29 inches long

A handy person from Ocala says:

I replaced a pair of channel balancers yesterday marked "28D USB". The windows I believe are identical so another question would why are they or how are they different, if at all.

And of course, what part number do I need, thanks!

Marked 28C, 29 inches long
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Tom from SWISCO responded:
Greetings! Please take a look at our S380-29 channel balance. You mentioned two different stamp markings in your inquiry. That stamp indicates your balance spring strength. In order to decide the best option, we recommend that you weigh your sash and correspond that number to our sash weight range chart listed on the web page.

Lastly, the top sash guide we are leaning most towards is our 18-180 .
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