Identify Window Channel Balance

A handy person from California says:
I need to identify a window channel balance. Please see the pictures attached. The total length minus the attachments is 21".

Thanks,
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Paul from SWISCO responded:

Your balance looks compatible with both our S351-21 and S390-21. The difference between these two balance lie within the bottom balance shoe. The depth of the bottom shoe for the Series 351 is 1" and for the Series 390 is 1-1/8". Measure the depth of your shoe like in the diagram below to determine which one would make the better match. Whichever one you go with will use a 2040 option. 

Comparison of 15-001 & 15-002
Comparison of 15-001 & 15-002
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