No number stamped on balance

A quick learner from Lutz Florida says:
November 17, 2016
I have no number stamped on the any of my balances. I checked all three windows. I think it's either 351 or 391. They look the same in the pictures.
User submitted photos of a window balance.
Profile picture of Paul
Paul from SWISCO responded:
November 17, 2016
I'm afraid I can't make out which Series would be a better match. The different comes down to the size of the shoe. We have detailed information and measurements on the S351-24 and S390-24 store pages that you can review. You will need to measure your shoe and use your best judgement, since the difference is subtle, but important.
24" Series 351 Channel Balance
24" Series 351 Channel Balance
S351-24
$10.77
24" Series 390 Channel Balance
24" Series 390 Channel Balance
S390-24
$10.55
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