Really need a better photo, but:
This looks like a UK produced US 1 sheet, style C. Double check the dimensions, 27" x 41" are the golden figures, although in this case that might not help.
There is no PG rating box, which is why I don't think this is a US or international version, but the location of the Original Motion Picture Box is not under the Star Wars logo, which could be the misprint you refer to, but more likely it is a proof.
This is tough to authenticate as back in the 80's an enterprising chap got hold of the original printing plates/parts and ran off thousands of these, very high quality stuff a that is where this miss placed motion picture box comes In to play. Being in the fame is a thing as well, as checking the edges would be good, if it is a proof then the edges might still be uncut, also the frame cuts off the tags often seen below the line on a fake.
If it is the real thing, and given the quality of the image I really have no idea, it very well could be, then you are up in the £1000-£1500 mark. Even if it is an 80's knock off, which was done with the correct dimensions, hence my early comment, you could still be in the hundreds, maybe £150-£200.
It is a great poster, one of the classics, perhaps you could tell us more, bit of history, better photo etc. that might help, all I can do is provide generic info, as for your poster I hope it is the real deal.
