1/2" Channel Balancer for non tilt window. Existing number 4964 BSI. Need less poundage.
How long are these balances? That stamp isn't making sense to me, unless these are absurdly big balances. Also, what poundage do you need? How much does the sash weigh? You may want to send photos of the attachments on your balance so that I can identify it.
To be honest, I actually have no idea how meaningful that number is. I was asking some preliminary questions, to get a feel for whether this is going to be a special custom balance.
I believe it is, and I'm not familiar with that branch of the industry. I'm going to flag down a custom expert to take a look at the information you already gave. In the meantime, gather up some measurements and photos of the balance, and come back.