A great resource for looking at originals is Flintlock Fowlers by Grinslade.
American fowlers generally fall into one of these categories: Hudson Valley fowlers, New England fowlers, British style fowlers, and “Kentucky” fowlers (almost always made in Pennsylvania). Each of these main regional types exhibit tremendous variation.
Guards are almost always pinned not screwed. A tang bolt typically runs from tang to trigger plate, but on very early ones, runs from bottom up.
The most fun for many of us is not making a generic fowler, but one in the style of an original or several closely related originals.