That is a great flask! Looks to be an early flask from a New England glasshouse to me - but I'm sure there are others here who can give you more info. Very, very nice!
Cindy, there is a shoo-fly flask from New England that looks very similar, but normally the necks are longer, and the lip is different... This one looks more like a Dutch flask...
I bought one knife edge coffin flask and a shoo fly similar to yours about 15 years ago out of a Heckler Auction.I sold the one similar to yours on ebay for $225.00.These were identified by Heckler as eastern flasks, I respect his judgement. I still have the other in my flask collection.