That's a good approach you suggest. I believe that these forgeries are contemporary, that is struck some time near the date on the coin. I doubt they are modern because a forgery that bad (i once heard a collector term it as "St. George and the Chicken" when i last found one on ebay), anyhow no serious collector would fall for that. Except Oli on this particular evening. In all fairness though Oli is not a sovereign collector and gold has not been his area. Whereas i used to collect sovereigns some time back before i sold them all, so naturally i've come across most of the 'modern' (1817-1932) gold coins from gold £5 coins down to the half sovereigns. And i've seen a few fakes like the one you picture being sold on ebay, i think yours is the third one i've come across. I saw one being sold as the real thing, it sold as far as i can tell, but you can bet some poor sod felt a bit of a fool after parting with however much they had done.