I bought a couple mounts with SO259 connectors included and already assembled. I had mocked up the antennas a couple times when I noticed the antenna connector was assembled incorrectly. I checked the other one I had bought and it was the same way.
The way it is assembled above the center conductor (the part doing the transmitting and carrying the power) is connected the antenna mount which more than likely will be connected to ground. Not good!
Here is the mount assembled correctly. The nylon washer is isolating the center conductor from the mounting bracket while at the same time the ground part of the connector is touching the bracket. My assumption is the connector was assembled overseas by unskilled labor.