Existing Balancers are missing, and I don't know what to get to replace them

Without having the original balance on hand, it will be very hard to identify what you need. We will do our best, though. I see you have our 18-171 top sash guide. This is popularly used with our Series 385 channel balance, though not exclusively so. I think it's a good place to start for a lot of reasons, but I'll go into that later. 

Before we go any further, we'll have to figure out what kind of Series 385 you need. Please tell me the height of the visible glass and the weight of the sash. That will allow us to determine what length and spring strength you need.

Perfect, thank you! I recommend our S395-27 channel balance with a 2650 stamp option for sash weighing 26 through 30 pounds.