Pete, there are a number of well studied early rifles that are of undetermined origin. This suggests that in the early years, there were few differences between colonial built rifles and some made at the same time in Europe. Of course we assume most of the time that if an early rifle is stocked in American walnut, cherry, or curly maple, this points to a colonial stocking of the gun. Certainly the furniture on the earliest colonial rifles does not differ much from that found on European guns. There are a number of early rifles strongly believed to have been stocked in the colonies for you to study. The Schreit rifle dated 1767 (if I recall correctly), the Marshall rifle, the Faber rifle, the "woods runner" rifle, the Fessler rifle (also known as the musicians rifle), and rifles 19 and 40 in Rifles of Colonial America are great guns to study. There are a number of others to study, and photos can be found in websites. Some of us have collected images of early rifles with colonial connections for years. You'll note a great deal of variety among the earliest rifles, with several commonalities. Early features include a wide and fairly tall buttplate with little to no curvature, a guard with the rail well off the wrist, a robust buttstock, and architecture that is comfortable for shooting. Contemporary makers who have mined this area and adhere closely to period examples include Jack Brooks, Mark Silver, Eric Kettenberg, Allen Martin, Mike Brooks, Mark Wheland, Mitch Yates, Tom Curran(Acer here) and a bunch more (don't want to get in trouble here).
If you're strictly looking to build a Christians Spring rifle, then the book on Moravian gunmakers by Steve Hench is affordable and indispensable.