In the 1770's it seems, you begin to see somewhat of a homogenization of German gun styles (and overall thinning in proportions, which seems to be rather sudden), and it becomes even harder to put locations on many of them (as if it was super easy all the time before...). My first thoughts were that this form originated in S.W. Germany, but on the other hand, I sometimes lean towards thinking N.W. Germany/Belgium/Flanders.... In fact, the more I think about it now, the more I think it is so. This gun styling seems to be primarily for smoothbore guns, not so much for rifles.
German/European guns are often unsigned, of any era. I may have seen guns of this style signed, but if I have, I don't recall. These are later than my period of interest, so I don't pay a whole lot of attention to them, honestly. I tend to just pass them by. Thin wrists turn me off.
I will say that my impression of these is that they ARE more "commercial" in appearance, if that makes any sense. They just look more like a factory-made gun, rather than crafted by the hands of artisans. Perhaps an over-romanticization, but still... I suppose that there was a general industrialization going on anyway, and it's only natural to start seeing this.
Engraving patterns sometimes far outlasted their stylishness. Old engravers die hard.
I think there is a general (and rather overwhelming) drive for people to want for things to be early. Early, early, early. They just really WANT the "thick" Isaac Haines guns to be pre-Revolutionary, for example (and I suppose they possibly could be... barely. And even that should help put a date on this hardware style, since, as I recall, the earliest known working date for Hains is 1773). And I think this desire to earlify everything colors people's thinking. Probably does mine too. In fact, my thinking may work backwards, as I refuse to believe something is "early" until I can see some proof of it!
The "Marshall" gun in particular, MAY be somewhat "earlier" than others of this basic form, I don't know. It doesn't have quite as much of an assembly-line feel to it, nor the very narrow wrist, and if someone found a signature and legitimate date on it of 1765, I would be only slightly surprised.
I went through my photos, and I can only find one gun of this basic style that I saved pics of. My apologies to whoever owns it, I have no idea where I got the pictures.
All of which is just my opinions, not something set in stone. It's all styles, forms, feelings, impressions... And it's been several years since I have made a rifle, or even barely touched a flintlock, and I'm a little rusty!