If your weight is unavailable, replace with 533051 - 985 SERIES SINGLE COIL BALANCE

PivotTrue® Fully Assembled Constant Force Coil Window Balance System, Single Spring Coil 

Each coil has a number, letter or both stamped near the beginning of the J-shaped spring/coil.
The number on coil identifies the weight range. Single coils have a size number of 3 -10. 
  
Some pivot lock shoes have a color coded stripe that indicates the weight range 
The xinc cam in the pivot lock shoe is designed to use a round head pivot bar, and will work with other shapes. 
Pivot bars are available with assorted lengths, shapes and hole patterns to accommodate various manufactures. 

Balance Shoe Width: 1-1/4 inch
Balance Shoe Length: 1-7/16 inch
Balance Shoe Thickness: 9/16 inch (1/2" Carrier)
Overall Length (Housing & 1 Coil): 2-5/8 inches
Housing Markings: PAT. # 6,983,531 - 5,371,971 

Coil Sizes/Numbers (will be stamped as a number, letter or both on the actual metal coil):
3A, supports 4-6 pounds when installed in pairs - may have brown stripe on bottom
4B, supports 7-8 pounds when installed in pairs - may have red stripe on bottom
5C, supports 9-10 pounds when installed in pairs - may have blue stripe on bottom
6D, supports 11-12 pounds when installed in pairs - may have yellow stripe on bottom
7E, supports 13-14 pounds when installed in pairs - may have green stripe on bottom
8F, supports 15-16 pounds when installed in pairs
9J, supports 17-18 pounds when installed in pairs
10K, supports 19-20 pounds when installed in pairs

How to calculate / choose the correct balance ( if you cannot read or find number on coil)
1. Weigh the window sash (The window sash is the part of the sliding window that moves up and down, it includes the frame and glass).
2. Divide the weight in half; this is the size for 1 side of the window.
3. Purchase 2 of the weights needed. Coil Balances should always be replaced in pairs.
Example: IF the window sash weights 24 pounds. You would need 2 12-pound coils, 1 on each side of the window.