Hello there!
We appreciate you reaching out to us. It appears you have either the Series 351 or Series 390, depending on the depth of the bottom fitting. Measure as shown below to determine the best match. There should also be a stamp imprinted on the metal channel that will indicate what strength option you'll need to select.
Has your window been working fine this whole time? If it has, you could just stick with the original 1340 stamp. You could also go with the 1350 stamp for a sash weighing 12 to 15 pounds if you feel that the window doesn't operate well with the original stamp. The decision is ultimately yours!
This window balance in these new photos is different from what you've shown in your initial post, however. What you have here is an extended travel channel balance which is only available by custom order. We can still help but I'll just need a few more details first.
The metal channel length should also end in a 5/8" increment. Custom orders are nonrefundable so you may want to check just to be safe.
