Window Block and Tackle

A handy person from 34667 says:
November 11, 2020
I have some Block and Tackle I need replaced. Best I can make out is they are BSI 21 12 from the imprint. I do however think that these are not the correct size for these windows as they are quite heavy. The window measures 4' wide by around 22" height. I have attached 5 pictures for reference. Thanks.
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Bob from SWISCO responded:
November 16, 2020

This looks like either an S351-22 or S390-22. The only difference between these two balances is the bottom nylon shoe depth, so compare both with your old hardware to see what matches best. Your stamp code of 21-12 is a little unusual. I recommend obtaining the exact window sash weight. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the shop page. 
22" Series 351 Channel Balance
22" Series 351 Channel Balance
S351-22
$9.91
22" Series 390 Channel Balance
22" Series 390 Channel Balance
S390-22
$9.59
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