Pattern 1863 Enfield-Whitworth Short Rifle of which something over 8,200 were manufactured during 1863 and 1864 for trials. The pictured rifle was manufactured in July 1864.
If you own such a rifle, or the 1862 Enfield-Whitworth, please get in touch. The underside of the barrel and back of breech plug have details including barrel supplier, date and serial number. Pictures would be much appreciated.