Your gun is most likely a Pattern 1853, 2nd Model Enfield rifle-musket. These were made in large numbers by numerous contractors for British use during the Crimean War. SOME of these guns also made it to America, and were used by both North and South during the WBTS. (Some were also actually made in America, before the WBTS.)
It can be difficult to tell whether or not an Enfield of this particular model saw use in America. Detailed markings analysis may reveal this, but not in every case.
The arrow pointing down on the lockplate is the Government acceptance mark - in other words, this gun was accepted by the British military (this arrow - a Broad Arrow - should have a small crown with it). In the case of 2nd Model P53s, this does NOT rule out later WBTS use. A number of these guns were later sold as obsolete and made their way into US and CS hands. However, a large number were refurbished and stayed in England. Again, the markings will sometimes reveal this.
Hope this helps,