Some additional reading on the topic. I certainly dont mind being questioned on this one as its a bit of a grey area and just like the white arm lobot there doesnt seem to be agreement.
http://forum.rebelscum.com/t904175/
http://www.imperialgunneryforum.com/t2261-unpainted-leia-boushh-helmet-help
I would be very hesitant about banding about the term prototype though as i believe there are only a very very small number of these out there and unless it came with a prototype figure or from a source of provenance then it should be taken with a pinch of salt.
In addition both the "prototype" and the "Lili Ledy overstock" are on Hong Kong issue helmets which are the same size and texture as the standard version and exceedingly easy to remove the paint from. Not that i have ever tried

It is therefore very hard to proove their legitamacy.
The generally accepted opinion, and I use that in its looses sense was that the true unpainted Boushh helmets came on Taiwan issue helmets which were larger in size and have a prominent raised circle on the inside as well as being very soft and rubbery texture and matte coloured.
The enlarged size of the helmet (and I'm feeling very self concious while I write this sentence

) is a good indicator of legitimacy although some have said this is a result of chemical stripping. However chemical stripping can lead to teraing of the plastic especially on the crown of the helmet which is quite noticable. I am also curious as to why it would not have a similar effect on the Hong Kong issue helmets.
So in my opinion a Taiwan issue helmet which is enlarged and made of a soft rubbery texture and matte colour and with a raised circle on the inside, with no damage to the crown and no paint residue visible is about as close as you can get to guaranteeing legitimacy. However, I would still only classify the real thing as a production error which in my opinion is a variant. Some might not call a production error a variant but many do and the terms applies in many other collecting circles so why not here.
My theory on the unpainted Taiwan helmet (and i'm no expert on the production process) so its purely a theory is that the type of material used and the temperature when it was removed from the mould may have caused the helmet to expand. It may have been difficult to paint these defective helmets with the paint masks so they remained unpainted but were still used on stock.
Judging by previous auctions i think £60 is a reasonable price. £250 is most certainly not and I dont think the term prototype is appropriate. From the photo it did look like a legitimate unpainted helmet though - just exceedingly over priced.
Mortlocks helmet was Hong Kong issue and came on a Tawian issue figure - so either way it was wrong plus he refused to even enter into conversation about its legitimacy.
I have some full length comparison photos to add, but no close ups at the moment until I get the light box out.
Oli