That looks a lot like the prototype helmet below.
Which would means, that it's a real preproduction piece.
http://www.starwarshelmets.com/unfinishedstormy.jpg
Above, an unfinished Stormtrooper from Andrew Ainsworth's personal collection, sold at Christie's in 2002 . Fabricated in white painted vacu-formed plastic with black rubber details, simulated vents and clear plastic eyepieces (one missing) -- made for the 1977 20th Century Fox/Lucasfilm production Star Wars; accompanied by a black and white photo of a group of Stormtrooper helmets and a corresponding black and white still. The above sold at Christies in December 2002 for $7,144. Looking at the photo it's missing its ears!
Read more about it here.
http://www.starwarshelmets.com/Original-stormtrooper-armor-helmets.htm
Here the original auction:
http://natedsanders.com/prototype-stormtrooper-helmet-from-the-first-sta-lot24520.aspx