Which option for my window.

Hello I have Lindford windows (they are no longer in business) and I need to replace my window balances as they have broke. Looking at your website I believe I need the S380-27 with 19-005 and 15-005.

Now for the option is where I'm not entirely sure. my window weighs 50 lbs (3 pane glass) I live next to an airport. Does that mean I should get the 2640 option or go for the 2650? The markings on the old rail is 26DS, BSI, OT, 36. I will enclose a picture.

What is their life? Mine didn't last 10 years. I found a local place here in Denver, but even though they promised their springs would hold my window, it did not past 1/3 open. Please let me know if you can help.

Please let me know your thoughts so I can order a pair to test and if it works properly, order springs for the entire house.
Miguel A.