29G op BSI sash balance

A handy person from Manhattan says:
Window sash is 40 lbs.
Chords broke. I have additional photos that may be helpful but could not attach more than 5.
The photo labeled DSCF3812 is actually of another window in the next room, just to show the bottom piece that connects to the post on the sash, as best I can tell.
This was a new build apartment in 2004 and we are the original owners; windows were installed with the new apartment, and thus are the original sash balances too. I'd love to find the original manufacturer to match the putty color of the sash balance, but haven't been able to determine who it is.
Please let me know if I can send the additional photos.
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Bob from SWISCO responded:

Thanks for writing. This looks like a Series 770 Channel Balance. However, this will likely need to be specially made due to the extra-strong spring strength and the weight of your window sash. Could you confirm for us the length of the metal channel? In the time being my colleagues and I will look this over and get back to you if we need any more information. Many thanks.

A handy person from Manhattan says:
Thanks Bob,
I was able to send the other photos in a subsequent form. It is 29". Is there any point in me just trying to restring it (since it's mostly the chord that's broken), or is that just not worth the hassle, given that these balances are not expensive?

I suppose we could reuse the springs from the old ones, but probably wouldn't make any sense. The springs are 16"

thanks,
frederick
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Bob from SWISCO responded:

Thanks for your patience during this busy season, Frederick. In general we never recommend trying to repair individual balances; this can actually be dangerous when it comes to the spring mechanism, as it is under a lot of tension. But in general, these balances are built as a singular component, and when they break its time to replace them wholesale. 

The good news is that we've added the stamp option of 29G for the S770-29 online. It will take 6 weeks for these to be made up, but you can sign up to be notified the moment its available through the S770-29 product page. Let me know if you have any other questions.

A handy person from Manhattan says:
Thanks, and happy 4th. I'm on the list to hear when the item is ready. I noticed that some online balance shops ask the buyer to ship the old one in for exact I.D. Would that be helpful, or can I be confident that this is the correct one, based on the stamp, measurements, and photos submitted?
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Bob from SWISCO responded:

I feel pretty confident that our S770-29 is the right balance for you, yes. The only thing you may want to double check is how it installs at the top of the window frame; ours uses a screw, but if yours uses some other method then let me know.

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