With the advent of nuclear power, he became known as Reddy Kilowatt, The Mighty Atom.
Ironically, he has SPIKES on his head -- although they are positioned more like Cobalt or Uran. I remember this guy and his name (Reddy Kilowatt) but NOT that he was ever called the Mighty Atom.
The coloring book is from 1959, and the other memorabilia appears to be from the early-mid 60's - exactly the same time Astro Boy premiered in the US. So there you go . . . Tezuka COULDN'T use "Mighty Atom" in America, because of Reddy Kilowatt. Otherwise, it's obvious he would have been know as Atom here, too!

Here's a link to a Yahoo group for fans of Reddy Kilowatt. I'm tempted to join and enlighten them to the REAL Mighty Atom :d evil:
http://groups.yahoo.com/group/themightyatom/
BUT LOOK!! The trademark HAS EXPIRED!!! YAAAY!! Astro CAN be Atom now!!

http://www.trademarkia.com/reddy-kilowatt-the-mighty-atom-72015903.html
And Reddy Kilowatt was the star of a education motion picture "The Mighty Atom"
http://advertisingiconmuseum.org/inside/c5/3237019.html