I'd go with the dipping method too, and my first thought were that it probably
is factory
I have seen something similar done, although not as intricately, on a mountain bike frame. That was done by dragging brush over the base coat whilst still wet, leaving a rippled/swirled surface. It was then blown over with the top coat and flatted back until the high points of the base were revealed through the top coat.
That example was done with a bright orange base, with the top coat fading from yellow to black, front to rear. Sounds wierd but the finished frame was
By the time it was finished nearly a quarter of the weight of the frame was paint