One thing they all have in common is that their made out of polymer, that sounds better than plastic doesn't it .Foxbat, you are going to have to do a better job picking on the New Frontier lowers. The top pic you posted was a Bushmaster, and the busted lower wasn't a New Frontier either. The New Frontier lowers have molded trigger guards, not pinned ones that you can remove. Just look at the pic in the first link.
The bottom is a Vulcan arms.
Vulcan Group Inc. [The Best-Built Firearms in the World!]