What resources are you seeing for bags dating prior to the 1840s? (That's a serious question: there just aren't a lot of bags that can be securely dated with actual provenance to use prior to 1840, vs "well, it looks old and they could have built it before 1840".)
Since Charles Hanson put me up to it in '81, I've been studying original bags used in the trans-Mississippi west. For about 25 years I spent a lot of time visiting local museums and private collections. I met a lot of neat people, saw some neat bags and horns, and while I still don't have any solid conclusions I did get some impressions of the common bags used in the west in the second half of the 18th century. All that doesn't make me "right" in any absolute sense, and those impressions are limited to a particular time and region.
My impression is that the surviving bags differ a fair amount from the bags commonly used today. No surprise there, as we use our bags differently than they were used then. I think on the whole, they also differ somewhat from surviving bags used in the same period in regions like Appalachia, or in the South. I'd say they also differ from bags used in earlier periods, but there just aren't a lot of earlier bags to compare to--mostly, we have period paintings and drawings, and the bags were seldom the focus of the illustrations.
From what I've seen, very few of the bags in museums and collections can be accurately dated in the sense that "Bob Jones bought this bag from a pedlar in Whachawhoosis County in 1849". We can sometimes go by makers' marks (e.g., a saddler's stamp known to have been in use during a certain period), or construction details such as machine sewing. Most of the time there is little provenance beyond "this was Grandpa Jones's bag, he died in 1901" or "I bought it at a farm auction in 1953, and don't know any more about it".
Construction-wise, many, if not most (I'd have to sort through my notes to be sure) of the surviving bags are machine sewn (vs handsewn, even professionally handsewn). Liners (or traces of liners) are extremely uncommon. Original fabric straps are extremely uncommon, but a lot of bags have fabric straps as replacements.
If I were trying to come up with a bag representative of those used in the trans-Mississippi west in the 1840-1860 period, I'd go with a simple commercial bag. Commercially veg-tanned leather of about 4 ounces, maybe some simple stamping on the flap. Machine sewn or handsewn, probably no welt, roughly 7-8 wide and tall. Flap sewn to bag body, with straps roughly 1-1.5" width sewn into the flap/body seam. If I was going to use a buckle on the strap, it would be a common harness buckle. I'd probably just oil the leather rather than dying it (it will darken over time on exposure to sunlight).
But that's just one man's opinion, based on bags with a history of use in a limited area.