While I'm anything but an expert on the topic of minting much less gold of this time period, it is my understanding that the mint marks were added to the die by hand, and that could explain the differing locations between coins. Given the number of coins struck this year in SF (2M+) it seems reasonable to assume more than one reverse die was used, and thus there would be some variation in the mintmark's location. Respectfully...Mike p.s. the above says nothing about the other (IMO valid) concerns about this coin's authenticity.
p.s. to clear the air on this, if the OP is interested, I will arrange for the coin be mailed to an expert who assess it and then (if he believes it to be real) to PCGS for grading -- and I will pay for all grading and mailing fees. Please PM me if you are interested.
Thank you Leadfoot, but PCGS is opening their office in Poland. It will be a lot easier to give it to them here than send it to you (a lot of customs paperwork needed).