Channel Balance Help

Looking at the two balances side by side, I don't see any differences that would prevent ours from working. 

When you say the balance is popping out of the window, do you mean that it's going up in the track without anything holding it down? If so, the top sash guide is almost certainly the culprit. I'd be happy to verify if the 18-008H is a viable option but I would need to see yours, first. A clear photo of the original in undamaged condition and removed from the window will be necessary. I specifically need to see it from the front and side angles; think of it like a mugshot. 

Perfect, thank you! Yes, I agree with your original assessment. The 18-008H should be perfectly applicable replacement for your original top sash guide. The differences are negligible.

The code doesn't verify what Series you have, unfortunately. That only tells you how strong the spring is. Up to this point I was just taking your word that the Series 385 was a match. If you have any doubts, you should measure the depth of the bottom fitting to verify. I can at least tell you, based on your photos, that this is the only difference you need to be concerned about at this point. Everything else looks okay but that shoe depth isn't something I can eyeball from here.

Comparison of the S378, S380 and S385 balance shoes.