How did they assemble the traps that have the locking lugs that keep the jaws from popping out?

Very carefully. Sorry, I couldn't help it.
I also would like to know as the only 42 Ranger I had was
broken. I was told you could bend them, but ....?

I always wondered that also

It is just a guess but I think that when the jaws were cast, the lugs were in the open position. Once the jaws were installed in the trap the lugs would be bent down to close them. Just like the cast swivels on Newhouse, Sargent and other traps, they are made open and when they put the chain on, it is closed. These also will break if you try to reopen them. It is a one shot deal. If you are very careful, you might be able to get them open but when you try to close it, it is just too much stress on the metal. It is like bending a wire back and forth, eventually, it will break. But, the wire is soft and stretches. But the cast jaws are stiff and made that way to keep them from bending, that makes them brittle. I do not recommend messing with them.

Sounds logical

That lug setup didn't catch on, for the reason
we're discussing. Triumph had many interesting
designs.

Sounds reasonable. Seems to be quite a few seen with broken lugs. I was hoping an old timer who knew an older timer who worked for Triumph would show up and tell us exactly how they did it.