Thanks for the reply Rob (and Paulus). It's been interesting researching the coin. I don't have many examples to compare - though I would love to buy more. My others are not nearly this nice and when I bought it I purposely wanted the best one I could afford. It still astounds me that a coin this old can look like it was struck yesterday. So having not thought twice about its authenticity when I purchased it I got a bit nervous after spending a little time with it.
In hand it's brighter, more silvery and the toning is very even. Although I know that there are some very good looking fakes on the market, after consideration the toning and patina seem right. Being in the US I don't see enough of these coins to have a good frame of reference. In some respects that's why I don't buy more (that and the expense).
Again, thanks so much.