Ah, the perennially popular question...what would an American rifle be like from the 1750's? Well, take everything you know about American gunsmithing "schools" and throw it out. Those schools did not really exist then. There are NO known American rifles from the 1750's and precious few suspected ones. A rifle of this time period would generally be VERY German in style or very English in style, depending basically on where you were from.
I'll assume you're talking about the Chambers "Mark Silver Virginia" rifle, which is closely based on the famed "Faeber" gun. PERSONALLY, I strongly suspect that this gun is from the 1760's. IF it had no cheekpiece, I would feel easier about it being called 1750's. This gun is extremely English in style, but with a German-type triggerguard and that horrible stuck-on cheekpiece. Lose the cheekpiece and I (PERSONALLY) would have no problem calling it a 1750's rifle.
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note: the above statements are solely the opinion of the author. they are not to be taken as anything other than personal opinions based upon the observations of the author. your opinion may vary. opinion void in MA, CA, IL, US Virgin Islands, Guam, the District of Columbia, and the cities of New York, NY, and Ottumwa, IA.