According to "Mazda MX-5 Miata: The Book of the World's Favourite Sportscar" by Brian Long and Takao Kijima, and "Mazda MX-5 Miata Roadster" by Toshihiko Hirai and Takaharu Koby Kobayakawa, this the "FR" mock-up made by Mazda North America in 1984 for the LWS (Light Weight Sportscar) project. You may find both sources on Google Books.
There is indeed one website that credits it to IAD but I think it's wrong. Apparently IAD was commissioned to make a functional prototype out of this proposal, but this static mockup was made without IAD input.