You are right with Your researches. It is a Hessen-Darmstadt sharpshooters rifle, a quite rare gun. It has a 13,9mm bore same as the Austrian Lorenz. Beginning on 26th April 1856 (Heidelberg conference)three South German states (Hessen-Darmstadt, Wuerttemberg and Baden),all members of the 8th Federal army corps of the German Federation) made an agreement for unification of armament. They finally agreed only in the calibre question of 13,9 mm = .54 = Austrian, relating to rifle muskets and rifles. Nevertheless the sharpshooter rifles produced (including Liechtenstein) were rather similar, due to economic causes. Only Baden produced a percussion breechloader (Terry system) in the same calibre, Wuerttemberg produced in Oberndorf, that gun is immediately identified having a front band of the French style (like a US 1816 musket). The Hessian gun is nearly identical to the Liechtenstein version (both made bei VCS) the Hessian having a safety device for the hammer like Yours. The Hessian guns were produced during the Civil War and were standard arms in Hessen-Darmstadt during the war of 1866 (between Austria, Hessen, Baden, Wuerttemberg, Bavaria and some other states against Prussia and its allies). So it is almost certain, that these were not sold to America. There is no evidence of any delivery of additional (commercial) guns for trade or open market. Guns used by Hessian forces have normally a stamp "GH" for Grossherzogtum Hessen-Darmstadt. Probably You are right with the theory of a sample gun. The only German sharpshooter rifles that could have been sold in a small number to the North may be the Prussian M/1835 ( a percussion conversion), as Prussia changed to needlefire guns at that time (but there is no proof of this theory). Also the fate of the older Hessen-Darmstadt equipment is totally unknown, as there is no evidence of deliveries to either North or South (perhaps a member of this forum has new informations about this).

Best regards,
Marc