1/2" x 28" replacement balance springs (2) for single-hung non-tilt

A contractor from California says:
Hi, These windows are appx. 25 yr. old, and there is no identification I could find on them.
I can't get the window out.... I think the plastic square blocks on each side, at the top of the window are maybe supposed to be popped out so the window can be slid over in the track...but I'm hoping you can recognize the manufacturer, by the hardware you see.
The only thing stamped on the track is the number 2740 followed by an upside-down "U".
Neither of the nylon end assemblies are wider than the track itself; they fit down into the channel with the track.
The balance spring I could get out is the one you're seeing, and the top pin, which held the top pulley, is destroyed.
This window does not have pull-out clips OR slots for them. Rather, it has lengths of square vinyl tubing blocks at the tops, but I can't easily get them out, and am afraid of breaking something.
The metal clip that should hook into the sash is, unfortunately, missing. Any advice?
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Casey from SWISCO responded:

Hello,

It looks like you have either our S351-28, with a 15-002 bottom attachment, or S390-28, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need a 2740 spring strength option for your replacement balances. 

1
It sounds like you also need new top sash guides that hold the balances in place below the window sash. There's a good chance that we can supply a replacement for your guide, but we would need to see one of your undamaged guides removed from the window, including dimensions, to compare to our inventory. Thank you!
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