It's a strap with a snap
We're too far from our trailer's storage location to get you a picture right now, but the original device was a leather strap attached to the door. There's a female snap at the lower end of the strap that fits the male snap attached to the screen door.
While the swing panel in the screen door is open, this strap can extend through and hold the two doors together.
Ours broke last summer, and I replaced it with a nylon strap with a snap.
Open both the door and the screen. Look in the inside surface of the door for a hole, rivet, screw or whatever used to hold the top end of that strap. Make a strap that extends from there to the snap in the door, install the appropriate size snap in the strap (found mine at a fabric store).
Sorry I couldn't find a picture. That would have been simple.