There were several other titles the show went through before it was christened MAZINGER Z, "Energer Z" being only one of them. I'm sure what everyone has said in response is exactly what came up at the brainstorming sessions with Toei Animation and Popy, before everything satisfactorily came together with the final designs which were approved of when the show aired as MAZINGER Z.
Too bad that there was also a huge strike at Toei Animation at the time, which really shows in DEVILMAN and the early episodes of MAZINGER Z. Still, this didn't effect the solid writing by Keisuke Fujikawa and others, and helped to make MAZINGER Z the sensation it was (along with composer Michiaki Watanabe's songs and score). While some of the recent remakes are brilliantly animated -- based on all of the original designs, the one thing that they lack is utilizing Watanabe's important music.
What book did this illustration come from? I have an earlier version of it, under a different title, in "Go Nagai: Design Sketch of Super Robots" (Media Works, 2003). I'm always looking for more books on Nagai's super robots.