"Eric, you hit the nail on the head. Original guns and even powder horns can be classified and assigned to an approximate place and time. Bags, not so much."
Not arguing with you at all Rich, regarding original bags, unless you know the person who had it. Certain bag/horn/rifle sets have made it down through the ages. That at least gives us a tiny window into "time and place".
If you are copying a bag from a painting of a known person, then you can attribute time and place. In other words, if you have a painting showing a 40-year-old Lord Stuffinsuch-Smythe from Wiltshire England with a particular bag, and you know his lifespan, you can give an approximate date. In that case I'd say something like "southern English influenced shooting bag from the 1790s" or similar.
We tend to talk American gear, and to some degree, I think some folks view American made bags as something created by a hill-billy mountain man around his fire using a pennyknife. There was a lot of importing going on in America's early history. I am a bit amused to see someone carrying a fairly crude Appalachian looking pounch with a Golden Age ornamented longrifle. I think someone in the day with the means to have a fine rifle would have had a fine bag too. Also, good sewing and repairing were pretty common skills that were needed back in everyday life.
I also believe that your coastal areas would have had a lot more English and European influence in the 1600s, 1700s, and early 1800s. It was cheaper to import goods along the coast than to make them locally in many cases. It was explained to me that is why there were few coastal gun makers in the Carolinas back in the 1700 and 1800s. English imports were so reasonable. Transport would have added a lot to the cost away from the coast.
So, I think we should keep firmly in mind that "where" was a big influence in "early" bags. For example, Louis the XV's shooting bag from 1750 would have looked a lot different than a poor Scots-Irish man scraping out a living in the back-country mountains of North Carolina. And both of their shooting bags would have looked different than Virginia's Lt Governor Robert Dinwiddie's bag of the same era in eastern Virginia.
My personal opinion is that vinegaroon is part of the reason we have so relatively few bags to view today. Rubbing acetic acid into leather does not seem a long-term solution. We can understand that now, but they may not have then.
Best wishes, and God Bless, Marc