Earlier guns had the three screw locks. Two screw locks use a woodscrew to secure the tail of the serpent.
Earlier guns had a tang screw that came up from below, threading into the tang, the head securing the trigger guard. Later guns had a machine screw from the top, which engaged a trigger plate.
The thin brass band found at the tip of the forend on some guns was a simple strip held in place by the barrel.
Northwest guns were made for a long time, by different contractors, for different companies.
Unless you are copying a specific gun, there are a lot of generalities.
Flint locks could be double, single or unbridled, depending on the company, the contractor and the time period.
NW Coy guns tended to have better locks than HBC guns - until the companies merged.
Same thing with buttplate nails vs screws; when, by whom, for whom.
Guns made for a specific company during a particular period tended to be quite uniform. The contractors produced them to a specified pattern.
The single best reference book is the one published by The Museum of the Fur Trade.