This was a feature found on many Jamestown School rifles. I would say that I have seen the double based front site on around 70% of all Jamestown Rifles. This feature and the long, rounded, three screw barrel tang are often the easiest way to identify a rifle from this school.
Over the years, I have seen a few Appalachian School or Tennessee Rifles that had a double brass dovetailed front site. I assume the maker had seen a Jamestown Rifle and liked that feature.
Michael Briggs