2840 channel

I'm not sure what you have, unfortunately. We have dozens of different balances with that stamp. It's not a product code or anything, it just indicates the strength of the spring.

In order to figure out what Series this is, I need you to remove an undamaged version of this balance from the window and show me clear pictures of the top and bottom nylon fittings. I will also need to know the dimensions of these fittings.

As for using alternative sizes, we always recommend sticking with the kind of hardware that's in the window to begin with except in cases where the original parts are obviously wrong or improperly installed. I'll be better able to advise you once I see what you have, though.