(I cannot write Italian, so I write in English. Please use auto-translation. Auto translation between Japanese and Italian is terrible, but between English and Italian is usable.)
Answers to Antonella Belli’s questions:
1) Description about Duke’s mother is in “テレビランドワンパック UFOロボグレンダイザー図鑑”(TV Land OnePack UFO Robot Grendizer pictorial book), Tokuma Syoten.。
On p.54, Duke’s mother is described as “デュークの母(実の母は昔死んで2番目のお母さん)”(Duke’s mother(second mother, real mother died a long time ago,)). This is the only source for Duke having two mothers.
The problem is the reliability of this material. Publication date of this book is June 15, 1971. Many pictures in this book are different from official design pictures. On p.59, the full name of Hayasi is written as “林恵三(Hayashi, Keizou)”. According to the episode No.12, Hayashi’s full name is Hayashi, Akira. I do not consider this document to be highly reliable. However, there is no evidence that this description is unofficial. The publisher is Tokuma Syoten, the same as the publisher of the magazine, テレビランド(TV Land) planned by Toei Animation..
2) Mr. Hanabusa write a detailed list of references in the booklet attached to the Japanese Grendizer DVD-BOX. I transcribed the literature part, edited in slightly, and put it on my website. Unfortunately, it’s only available in Japanese at the moment, but you can use automatic translation with these pages.