"That one was discussed here some years ago and Artificer pointed out that the flap design was the same as a Hessian boot (Hessian boots in this context indicating a style of footwear, not just one worn by German dragoons during the Revolution). He suggested that it was probably either a recycled boot or possibly a practice piece by an apprentice cobbler, IIRC. A bit of research indicates that boots of this design only became fashionable after 1790 or so, and evidently were still in use until after the Civil War in the US (though they fell from favor after 1830 or so in Britain, I think). I'd say that was most likely a 19th century piece."
Elnathan, I did some looking online, and it does appear to me that at least the flap is from an actual Hessian boot. It appears to be the top of the boot, inverted, opened down the back seam and laid open. I noticed that those Hessian boots had the "pull tabs" on the sides, so I zoomed into the photo here. Yep, on each side you can see a line of stitching from tabs from its previous use. If it was from an apprentice, I would expect it to lack those stitch lines.
Not sure why they cut the "pent roof" top line versus straight, but it sure looks like an upside down boot top. Maybe the rest of the bag is from boots too, but I cannot see enough to say.
I looked for that "mustache" line of stitching on boots, and could not find it, but that means nothing. Could be a slightly dressed-up variation, or could have been added when a boot was reused as a flap.
From my limited research, it sounds like some limited Hessians (officers, horsemen??) may have used them in the Revolution, hence appearing on the English radar screen so to speak. They became a British Regency Period fashion statement as you noted, and were later called Wellingtons. So it is possible that the bag could be from a wartime Hessian boot, re-used. As you also noted, it would be a safer bet to say 19th Century.
Thank you for sharing that with us. Very cool. God Bless, Marc