channel balance replacement

A quick learner from US says:
May 20, 2020
Hi,
I'm hoping that you can help me source some replacement window balances. The part I have in place right now appears to be a BSI 31TGX. I can't find this one for sale anywhere on the Internet, but I did find reference to it in your help section here:
https://www.swisco.com/discussions/32-Channel-Balance-77-52864
and I found pics of a similar-seeming application here:
https://www.swisco.com/discussions/BSI-balance-replacement-parts-77-56169
These balances are used in a modern low-e window retrofit into an historic building for which modern plastic channels have been mounted onto the face of the original window jamb (removing the original sash weights), and then the channel balances fit into the channels.

Below I've added numerous pics, which I hope explain the issue. I'm sorry if there's too much, but better safe than sorry.
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Paul from SWISCO responded:
May 25, 2020

Hello,

It looks like you need an S751-32 ribbed channel balance to replace your originals. The spring weight code will be (31TG) based on your balance stamp. Also, make sure to review our video on installing these balances and let us know if you have any other questions!
Long S-Clip
Long S-Clip
19-030
$0.70
32" Series 751 Channel Balance
32" Series 751 Channel Balance
S751-32
$12.27
How to install a ribbed tilt channel balance with knife lock shoe
How to install a ribbed tilt channel balance with knife lock shoe
VIDEO
WATCH >
Classifying Channel Balance Fittings
Classifying Channel Balance Fittings
GUIDE
READ >
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