Series 770 Balance Replacement
A quick learner from Balances says:
March 3, 2014
My existing balances are stamped '2800 BSI LF' Based off that data can you determine what the weight rating is? Weighing the window sash is going to be difficult.
Tom from SWISCO responded:
March 4, 2014
I'm afraid I'm not familiar with that stamp. I think weighing the sash is your best bet at this point.