Please identify my balance and what is stamp 27 C BSI?

A quick learner from Jasper, TN says:
This balance is 28" long and is in an Alenco window. The stamp says 27 C B.S.I.

Is the 390 series what I need?
User submitted photo of their Alenco window channel balance.
Profile picture of Donna
Donna from SWISCO responded:
Yes, SWISCO Series 390 balances are needed.

The 27 C normally translates to 2730. To verify stamp, remove the window sash and place it on a scale. A 28" Series 390 with stamp number 2730 will carry a window sash weight range of 13 through and including 19 pounds.

This information is based on 2 channel balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
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