Casey from SWISCO responded:
January 13, 2023
Hello, It's hard to tell based on your photos but as long as your top and bottom balance fittings do not have wings, you could have either our S351-21, with a 15-002 bottom attachment, or S390-21, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need to weigh your sash to determine which option you'll need since your stamp is from an outdated weight system. There is a chart on the product page that explains how to choose your option based on the sash weight. Comparison of the S351 and S390 balance shoes.
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