Well, considering i was the one who sold it to him, i'm sure i can best answer your question.
When in Mexico City in 2004, i found a seller who had a lot of factory overstock pieces from the Auriken factory, which was the company who manufactured GI Joes in Mexico under license from Hasbro during the 1980s. I bought a number of unpainted, overstock heads from him, which you can see here:
While these items could technically fall under the pre-production banner, they aren't prototypes or first shots by any means - they are simply overstock or unused production pieces and rejected parts (not to say that this one was rejected, but one of the ones in the batch that i bought was clearly short-shot, something that happens when not enough plastic is injected into the mold. When this happens, the parts are rejected).
So, while they are not an actual part of the pre-production process, i think they're interesting in their own right as production overstock.
I sold this one to Marc last year some time, for about the same price you paid and with the exact description i just gave you. I guess he added a modern Storm Shadow body to the figure for display purposes but it definitely does NOT go with that head.
So, now you know the whole story of where your Storm Shadow head came from. I hope it helps answer any questions you had and if you have more, please feel free to ask here or contact me via PM.