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A quick learner from Riverside nj says:
June 28, 2020
1/2 x 21 balancer with2010 marked on it. The S351 or S390 look like they match. Can you tell me which one I must order. Thank you
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Casey from SWISCO responded:
July 1, 2020

I agree. To choose between the two balances, take a look at the diagram below and measure to determine if you have a 1" or 1-1/8" deep bottom attachment. Thank you!

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
A quick learner from Riverside nj says:
July 1, 2020
It looks like I have an inch and 3/16 So I assume I use an S390. Thank you
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Casey from SWISCO responded:
July 1, 2020

The shoe can wear down a bit over time but if it measures over 1", I would agree that the s390 would be the way to go. 
21" Series 351 Channel Balance
21" Series 351 Channel Balance
S351-21
$9.59
21" Series 390 Channel Balance
21" Series 390 Channel Balance
S390-21
$9.38
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