channel balance replacement

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.