Emily from SWISCO responded:
July 26, 2023
Hello! Your balance looks like either the S380-20 or S385-20 to me. You will have to measure as shown below to determine which Series is best. The 19-D-40 stamp may correspond to our 1940 stamp option but you can weigh the window sash on a scale to confirm. A pair of balances with this stamp can support a sash weighing 14 to 17 pounds. Comparison of the S380 and S385 balance shoes.
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