Window balance need identifying

A quick learner says:
March 3, 2021
The metal part of the balance is 35". The pivot shoe is 1 1/2" wide, 2 1/16" tall 1/2" thick. 3500 is stamped on the balance.
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Bob from SWISCO responded:
March 4, 2021

Hi there. This looks like a S770-35 Channel Balance and 15-129 Pivot Shoe. Make sure your balance attaches to the top of the window jamb with a screw; if it uses something else like a hook, please show me. To know what stamp option to select, obtain the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the balance's shop page. 
Pivot Shoe, 9/16" Thick
Pivot Shoe, 9/16" Thick
15-129
$2.50
35" Series 770 Balance
35" Series 770 Balance
S770-35
$13.82
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