How do I secure wiring in an exterior electrical box?
What you see is very common, no RX connector. I think you will find that a common $0.50 3/4 inch romex (NM) connector will screw directly into the treads of the knock out.  



Simply remove the tork nut and thread the connector into the box hole threads.  This will work fine with the back hole since your going to seal the box to the wall with silicon.  If you were to use the side, top or bottom knock outs, or not otherwise give the opening a water tight seal, you would have to use water tight connectors for the wire size.( rubber insert for 12/2, 14/2 UF/NM Etc.