Hi,
Keep in mind that New England fowlers varied a lot. Styles from eastern Mass differed from the CT River Valley, as did guns from western CT. Grinslade's book is pretty good but the photos are mediocre. The best strategy for NE fowlers is to visit museums and collections. If you want to really learn about 18th century NE guns visit Old Sturbridge Village in MA, Memorial Hall and Flynt Museum of Decorative Arts at Historic Deerfield, MA, and Fort Ticonderoga in NY. Those are great places to start.
dave