It's just another broken Cat boom then. The placement of the break is what I've commonly seen in the past. Usually the crack shows up to start with as a spider line of rust that starts in the upper corner and runs both directions, across the top and down the side. The break didn't happen all at once. It's been insidiously growing over some months. There were product bulletins out for years and on multiple models about checking for cracks. It's good practice not to weld across a boom. If you have to weld something on like a tube clamp, you should weld in line with the boom. I wouldn't hazard a guess if that had anything to do with this failure. I've seen enough of them to say it was more likely a design or quality issue. At the dealer I was working for, they didn't bother trying to fix something that went that far. I don't know the availability of a used boom, so you might have no choice but to fix it.