What option do I need for a 24 B stamp?

A home owner from Baltimore, MD says:
May 11, 2015
The channel balance I want to replace has the code 24 B stamped on it. It looks like the S370 and is 25". I am not sure which stamp option to choose or the sash weigh range. Can you tell from the " 24 B" stamp?
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Paul from SWISCO responded:
May 12, 2015
An option of 24 B commonly corresponds to our 2420 option. Two balances with the 2420 option can carry a sash weight of 9 through 15 pounds. I recommend weighing your sash to confirm that this will work for you.
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