Looks to me like it is a remnant of a flange that would have allowed brakes to be installed. Does it look like it has been torched off? If the '21' is an indication of the boat size that the trailer would accomodate, it surely should have at least been a brake capable axle.
it does look like the brake flange, I would look at where the spindle is welded into the axle tube and make sure it's ok, wire brush it good first. Otherwise it should be fine.
