I do not know how the original ones were manufactured. However, I recently purchased the aluminum reproduction version from gas pump heaven. It was manufactured by using the "spinning" process. It's a way of making a rounded surface out of a flat piece of metal, sort of like how a person uses a spinning wheel to form a clay pot. I'm certain they used the spinning process because you could see fairly deep grooves around the circumference of the globe where the wheel was contacting the metal. The grooves were big enough to see and feel with your hand. You could not just paint it, you had to coat it with body filler first, then wet sand.
So, look inside the globe, maybe you'll see those spinning marks on the inside where they didn't use body filler, or maybe they even didn't paint the inside.