How to read the coil balance spring number (reading up to the hole - 6 or 9)?

A home owner from Centreville, VA 20120 says:
April 18, 2017
I attach the picture of coil. I'm thinking of number 6, but please check and confirm for me. My windows size outer of glass is W 31' x L 38".
User submitted photos of a window balance.
Profile picture of Tom
Tom from SWISCO responded:
April 20, 2017
Thanks for posting. I would recommend our 15-180 pivot shoe and S190-640 coil spring.
640 S190 Coil Balance Spring
640 S190 Coil Balance Spring
S190-640
$9.74
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