I doubt its the work of a fan. I've been watching the Prometheus making of documentary, Furious Gods, and all the screenshots of Spaight's draft matches up perfectly with this copy. If it's a fake, it must have been a very intricately researched one, because as far as I'm concerned, it seems pretty real.
- Certain shots of the script in documentary match up word for word
- There's the mention of a 5 spoke wheel of a space station Weyland resides in and a round shuttlecraft Holloway and Watts take - both of these are seen in the form of concept art in the documentary, with the former being specifically described by the concept artists as a Weyland station.
- Hammerpede is described as a centipede like creature with a hammer like head and a slit in the mouth that becomes horizontal when it's about to attack - this centipede like hammerpede concept art is seen in the documentary and there's a Ridleygram briefly shown depicting a hammerpede with a horizontal mouth.
- Older Holloway, squishy white aliens, David playing with the hugger, links to Alien, sex-burster, Milburn and Fifield losing the maps, have all been mentioned in interviews with Spaights on the draft.
Nothing faulty so far - safe to say it's legit.