Window balance finder

A quick learner from USA says:
Per our conversation, I am submitting the attached photos for identification purposes.
The balance is stamped" 1720", 18x12"
User submitted photos of a window balance.
Profile picture of Tom
Tom from SWISCO responded:
I believe you have either our Series 351 or Series 390 channel balance. The only difference between these two is the size of the bottom shoe, shown in your first picture. Look over these options to determine the closer match.
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