Window channel balance question

A handy person from California says:
December 23, 2020
Hi, my stamp says W2640, while I think I have narrowed down the series to two possibilities, I am not confident as to whether it is series 390 or 351.

Are you able to help based on the picture?

Thanks,
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Bob from SWISCO responded:
December 28, 2020

Thanks for posting. The difference between the Series 351 and Series 390 is the bottom nylon shoe, as you can see in the illustration below. A stamp of 2640 would indicate you need a 27" long balance, but double-check to confirm. Let me know if you have further questions. 

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
Channel Balance Top Guide
Channel Balance Top Guide
19-002
$0.74
Channel Balance Top, Winged
Channel Balance Top, Winged
19-005
$0.74
How to Replace a Channel Balance Attachment
How to Replace a Channel Balance Attachment
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Classifying Channel Balance Fittings
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