Another channel balance I need help to identify

A quick learner from Tampa Bay Florida says:
I have channel balances 29" long metal (not including plastic ends)
I don't have window measurements, but I do have the screen measurements for that window: 31" tall and 49 1/2" wide.
The only stamps in the channel metal that I can see are "28C USB".
Please advise which channel balances I need to order so I can replace.
Thank you!!
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Emily from SWISCO responded:

Hi, Marc.

With this combination of attachments, you could have either an S380-29 or S385-29 channel balance. You’ll need to measure as shown below. The Series 380 has a 1" bottom attachment, and the Series 385 has a 1-3/32" attachment. We also suggest weighing your sash on a scale to determine the appropriate spring strength option. There's a chart on the S380 and S385's store pages that will tell you which option you'll need based on the weight of your sash. 

If you have any questions just let us know!

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