What a great find that is. I have spent many hours on early French sites and I kid you not it has literally taken me years just to get a small higher grade French silver coin. (And I was very happy with it)
http://forum.treasurenet.com/index.php/topic,273062.0.html
Digging an ECU, now that would be off the charts. Colonial French silver does not get it's just due, especially when comparing Colonial Spanish, but I can say with certainty an ECU is many MANY times as rare to dig as an 8 reale. I'm not even sure If I have seen another posted from North America. If only the book value had to do with how rare it is to dig!
PS... Better find a new expert

It was definitely minted in France and you can find out exactly where because the strange symbol is the mint mark. It's very late here, but if no one tells you where in France it was made I'll tell you later today.