It can be confusing, yes it's the part of the connector above the threading, which has the little slots for venting on a 510 anyway. You could also touch it to the inside of the threads it's the same thing.
A easy way to tell the difference between positive and negative on the connector is look down in it. you'll see the center pin, that's the flat part in the middle(positive). Surrounding that will be a little gasket which is what separates positive from negative. Outside of that gasket is negative.