If this is the piece that holds the door open so that it will not swing back to close it is called and "outside main door hold back". It is made up of four pieces not counting the pop rivets. I beleive you want the little catch that is attached to the door itself by two pop rivets. The other pieces attach to the body of the trailer outside and consist of a plastic spring loader plunger and an aluminum shell the plunger slides back and forth in mounted on a plastic base. You can order each piece separately and they are not expensive. www.odmrv.com
carries them. I know becuase I have a new one on the way right now because a gust of wind caught the door and slammed it against the latch hard enough to snap the tapered part off the sliding plunger. If I were you I would order one of those as well to have on hand along with the door catch portion. This is the second time I had to replace the thing for the same reason.