Tom from SWISCO responded:
May 25, 2017
I'm not sure what you mean by prongs. Do you mean the nylon attachments on either end? Your top one looks like our 19-002, while the bottom one looks like our 15-001. Like you said, that would match our Series 390 balance. Can you also confirm the length of the metal channel? If the stamp reads 3020, it should be 31" long, not 30".
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Tom from SWISCO responded:
May 26, 2017
It is very strange for a balance with a 3020 stamp to be 30" long. I wonder if they were cut down on-site when the windows were installed. It's been known to happen, if the installer feels its necessary. Either way, if you have the S390-30, I would recommend weighing your sash on a scale to confirm the best stamp option.
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