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This is the manual for the retail release of the original Doom, sold as Ultimate Doom, containing a new episode.
I find myself wondering just about the nature of this cover art, because it doesn't really look like any of the concept art Adrian Carmack drew for Doom, and it doesn't look like any of the clay models he sculpted, nor like any of the steel-latex models which Gregor Punchatz made for iD Sofwatre to digitize.
The lighting of it makes it look rather realistic though, like it was a real life sculpt or other model of some sort.