Stamp option Series 390 Channel Balance

A professional from California says:
I have narrowed down the type to Series 390 but cannot tell from the stamp which number represents the stamp option. I have attached a couple of photos of two different balances I need replacements for. Please advise which number will help me determine the stamp option.

Thank you,
User submitted photos of a window balance.
Profile picture of Tom
Tom from SWISCO responded:
Hey, Jim. A stamp of "27 5" usually matches our 2750 option, while a "21 4" stamp matches our 2140 option. You can also weigh each sash to confirm.
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