stank,
I can't definitively answer you request, but I can give you my impressions of your trade gun. I think it is likely a vintage kit gun. My rationale is the poorer quality of the build and the amateur attempt to mimic proof and view marks on the barrel as shown in this picture.
You say the bore is .62 caliber which is equivalent to 20 gauge. I don't understand why the barrel has "16" stamped on it. Around the middle of the 19th century, English law required the bore or "balls-to-the-pound" be marked on all gun barrels. NW trade guns after that point had the bore size stamped on the barrel. If the builder was trying to mimic this, then why stamp it with "16" rather than "20" or more correctly "22"? I suppose the number could represent the 16th gun built by the builder. The other marks near the number do not mimic any official proof and view marks and are either fanciful or part of the custom builder's makers mark.
The lock panels are too wide and the beaver tail at the back is poorly shaped and too large. Some of the inletting is amateurish such as the front of the lock and the tang appears to be proud of the wood.
Back in the 1970s and 1980s, there were two small companies producing North West trade guns and kits. These were Green River Forge which was started by Frank Straight and North Star Enterprises started by Curley Gostomski. I have a Green River Forge NW trade gun, and it is different from yours in a number of areas. I don't own one of Curley's guns, but there is a picture of one on page 42 of
The Book of Buckskinning III that has a lock similar to yours. Gostomski's locks and barrels usually had a "Circle Fox" stamped on them. Your lock is unlike any that suppliers like Track of the Wolf was selling in the 70s and 80s, suggesting a proprietary casting.
Primarily based on the lock, I believe your trade gun was probably built from one of Gostomski's kits.
Curley eventually sold his business and the name was changed to North Star West. It finally shut down two or three years ago. It's possible the kit and gun could be from North Star West.