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Bob from SWISCO responded:
February 17, 2021
Hello there. Let's see if I can help. Are these the original balances? If so, it looks like you have what we call either Series 380 or Series 385 Channel Balances. The only difference between these two balance Series is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to determine which you need. The image below depicts the difference. You then have to choose a stamp option. The stamp option is what designates the weight range the balance is calibrated to support. A stamp code of 29-9 is from an older rating system not in use. I wonder if the replacement balances you purchased were for a weight rating too weak for your windows; hence why they're still too difficult to open. I advise you obtain the exact weight of your window sash. You can then match that exact weight with one of the weight ranges we list alongside each stamp option on any given balance's shop page. 
Comparison of the S380 and S385 balance shoes.
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