Hi There, welcome to the world of the mysterious sash balance! If you go here:
http://www.whitco.com.au/products/pr...Name=more+info
and click on Window balance systems you will get a pdf showing how to derive a Whitco product code for the particular balance your window requires. Once you have that number, you can go along to most hardware stores that have a special orders desk and order it in, eg. Bunnies.
You will need to do a few measurements and cross reference to the table on page 4, the code is then assembled as per the example at the very bottom of page 3.
Cheers
Michael