Early prototypes of the iPad in this article by NetworkWork. 9to5Mac pointed to this article and has an extra interview with Steve Jobs.
iOnApple: Earliest known photos of an Apple iPad prototype: “Earliest known photos of an Apple iPad prototype
Well before the iPhone, and going back all the way to the early 2000s, Apple had been working on tablet prototypes that would eventually form the groundwork for the original iPad
By Yoni Heisler on Wed, 07/18/12 – 9:32am.”
Ref to 9to5 Mac article.