The Whitney-Remington used a different type of pin retainer; that gun appears to be a pretty standard Remington rolling block, and it most likely has the Remington name on the upper tang.
It could be any of the many thousands of military rifles turned out by Remington and a few foreign factories. The sight rules out 7mm, and appears to be for a far less powerful round. Check the markings and also if it appears to be military or a cut down military.
Jim