Above are two rumored pictures of the Wii U’s first dev kits, codenamed under the CAT-DEV (Cafe Tool for Development) banner.
The hardware lacks DRC (Display Remote Controller) support and is without any optical media. Developers were only able to make use of the Wii Classic Controller at this stage. The back of the hardware features the same outputs as the Wii along with the HDMI output/the classic controller port.
Of course, future iterations introduced new functionality and upgrades. Without DRC, I don’t believe developers would have been able to make use of the Wii U GamePad.