Window balance

Good Afternoon.
 
We had the string break on the balance for one of our windows so I am trying to decide which is the correct balance.  The metal part of the balance measures 25 inches and is stamped 2420.  In looking at the options you offer the closest that I have been able to find is product code S390-25.  However I do have a couple concerns.  First, on both yours and ours the plastic piece on the pulley end of the balance measures 1 1/8 inches but yours is not quite the same as ours.  My second concern has to do where the spring is attached in the balance.  On ours it is down a bit from the top but on yours it is right at the top.  You will find 4 pictures attached that show exactly which balance we have.  I am assuming if this is the correct replacement balance we should order 2 so both sides are the same.  Correct?  Thanks for your help with this. 

Hi Bob.  Just wanted to follow-up on one of my previous questions.  The S350-25 does look very similiar to the balance we have but am still wondering if I would need to order 2 of them so they are a matched set on the window.  Just not sure if where the spring is attached between the old and new balances would make a difference in how they operate.  Thoughts?  Thanks.  David