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Tom from SWISCO responded:
September 1, 2017
This looks like either an S380-26 or an S385-26 with a 2540 stamp option. You can verify the length by measuring the metal, as it appears you did, and you can verify the option by weighing the window. It should weigh between 19 and 25 pounds. As for the Series, measure the bottom shoe as shown below and choose the proper balance. Series 380 has a 1" shoe, and Series 385 has the 1-3/32" shoe in the picture.
Comparison of the S380 and S385 balance shoes.
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Tom from SWISCO responded:
October 10, 2017
If your old stamp was 2540 and you found it worked fine, then you could stick with it. That said, I'd usually recommend 2530 for an 18.5 pound sash just because the minimum sash weight that the 2540 option can support exceeds the weight of your sash. A stronger spring isn't always better. But again, if you found 2540 worked fine in the past then you can stick with it if you prefer.
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