Here ya go.....
Bottom trap is non-mod. It has a single frame. Top trap has a second strap riveted to the frame. Also notice how the bottom trap's frame is bent. The chain is attached in the center of the frame, and as they lunge, the frame bends. Once it bends enough, it allows the jaws to be pulled out of the hinges.
Second mod is the laminations on the jaws. It keeps the jaws from being bent, and pulled out. The laminations also add more surface area on the jaw faze. Really helps hold magically caught yotes.
This is something that has always bugged me, and actually gives the trap away on occasion. The loose jaw (front), sits up higher than back jaw, and the pan. If a coyote doesn't step right on the pan, that jaw is up higher and they can feel it. Its also harder to bed properly, The four coils come from the factory that way.
I tack a small weld on the back jaw so that it pushes down on the spring levers, and it then drops down level. Like this.
You can buy the traps modified. I have a few left that I will modify myself in the off season. The only other thing I do is put an inline spring (shock) on the chain. You can see it on the top picture. In spite of all that, they still bend the dogs, and tear them up on occasion.