replacing spring balance 47.5"

I'm not sure if you can use our Series 600 or not. You said that the diameter on yours is 11/16", which is already a red flag. That's huge; no spiral balance has ever been made in that size.  Especially if it's 47-1/2" long on top of that.

Rather than guess, I think it's best if you show me photos of your balance. That way we can be on the same page and I'll be better able to advise you.

Ah, I recognize this. This looks like the special Hybrid balance. These are generally used in heavy duty commercial windows, like in schools and stuff, but we do see them in residential homes sometimes. This appears to be one of those times.

The diameter of the outer tube should be 1/2", and there should also be a stamp printed on the tube itself near the top (where the screw is). Can you find that stamp for me? It will tell you the sash weight without you having to weigh it.

If you can't find any stamp, though, then weighing the sash on a scale is your only option. These are calibrated very precisely, down to the ounce in some cases, so you can't really avoid that. Ideally the stamp on the tube will still be readable, so you can use that, but if not then you don't have another option.

You're right, that is the Series 600. To answer your question, this is not common at all. In fact it's very weird. Someone must have repaired this window in the past and mixed and matched repair parts. But that just begs the question: which one was original to the window? The Hybrid balance or the Series 600?

We can sell both to you, but at this point I don't know which one is the right one to go with. It honestly just complicates matters way more than they were originally. Do you have an identical window in the house that might still have all its original parts? You could use that as a reference. Otherwise, you will have to be the one to make the judgement call. You could try the standard Series 600 and see how well that works. They can always be returned for a refund, after all, which the Hybrid balances cannot.

We sell both, though. It's ultimately up to you. There is unfortunately no easy way to get confirm the right way to go. Someone repaired this window in the past and used the wrong parts to do so. We're kind of just cleaning up their mess at this point.

I do believe they both use a #8 screw, though they are not included. I'm also not sure what length you need; that depends less on the balance and more on the window. I would recommend taking the original screws to a local hardware store to have them match it up.

If all your windows are designed this way then they must have had a good reason for it. We may as well stick with the mixed and matched Series 600 and Hybrid balance. If there is no stamp on your Hybrid tubes, we will need the exact sash weight, down to the ounce if possible. These are very precise systems.