Hello,
This is a SWISCO Series 390 balance.
Please check on the length of the balance, exclude fittings for this measure, I believe it may measure 31".
Next, we need the weight of the window sash. Please remove the window sash and place it on a scale to get us the weight in pounds.
You were correct, it was 31" and the weight of the window sash was 25 lbs.
Thank you.
It seems your replacement will be the S390-31 Channel Balance, with the 3040 stamp.