The ShodaiAngira's name comes from shodai (初代?), meaning first generation, and Angira, which comes from Anguirus's Japanese name, Angirasu (アンギラス?).
The ShodaiAngira is thinner and has a slightly bigger nose than the later suits. During filming, the carapace flopped up and down. A notable detail on the carapace is that the spikes are placed in straight rows.
It would seem that this suit's floppy carapace was an intentional feature of Anguirus' design. Subsequent concept art for the unmade film, Godzilla vs. Ghost Godzilla, shows a design for Anguirus with the halves of his carapace being able to move around.