Tohl Narita: (about Iron Rocks) “Just like Dino-Tank, this is a strange one as it was the kaiju from Battleship Yamato. This is a design I drew when I was thinking seriously of resigning Tsuburaya Productions.”
As the design drawing of Iron Rocks is included in neither of Tohl Narita’s art books, it is uncertain whether a design drawing by Narita did exist or not.
Although it had been said for a long time that the prop used for Iron Rocks was one of the Battleship Yamato props used in a Toho movie, it was revealed quite recently that the Iron Rocks prop was genuinely made by the sculpting section of Tsuburaya Productions, most likely, from scratch without using the Toho prop.
While I had received an impression of softness from its material in appearance, it seems that the prop was made from latex cast out of the mold and was reinforced with FRP from inside.
But it is told that they estimated the rigidity according to the size wrongly and that the whole thing collapsed as soon as it was taken out of the mold. The sculpture of Iron Rocks seems to have been disapproved by Eiji Tsuburaya either when he was on the set saying, “Such soft stuff doesn’t work out.”
It is likely that the prop was supported by thick lauan lumber materials from inside in the end.