Can't tell which balancer

I can't tell either, I'm afraid. All the stamp tells us is how strong the spring is. In order to confirm the best match among the Series we carry, you will need to carefully compare our measurements, pictures, and information to your own hardware.

Well, let me go over how these attachments work, a little.

You have two main varieties: winged and non-winged. Between the two Series you use as example, the Series 380 and Series 390 balances, the only difference is that the Series 380 has wings on the attachments while the Series 390 doesn't. What these wings do is stabilize the balance in wider tracks. Some tracks aren't so wide, making the Series 390 work fine, but for those instances where it's a little larger the wings on the Series 380 keep it from wobbling around while you raise and lower the window.

Another key difference is the depth of the shoe. We'll use the Series 380 and Series 385 as examples. The Series 380 has a 1" deep shoe while the Series 385 has a 1-3/32" deep shoe. This may seem like a small difference, but if your shoe is too deep it will make the window too tight in the frame and could even cause damage to the sash.

Hopefully this helps you narrow down your decision.