Oh, boy! I thought at first it was one of the Burton patent guns because of the shape of the bolt handle, but those used an interrupted thread, not a single locking lug. The original Burton patent was for a conversion of muskets to cartridge loading, so you might check to see if there is any indication of that in your gun. (The large bore could be one indication, perhaps it started out as a .69 caliber musket.)
It certainly appears to be military from the stock and markings; the set trigger indicates it was probably a sniper or sharpshooter's rifle.
I suggest examining the barrel and breech carefully, and removing the barrelled action from the stock, to see if there are proof marks. If there are, we might at least determine the country of origin.
Jim