Your "BACKMARK" dates from as early as 1850 to 1865. The back, although at first I was doubtful, is the 'Name in groove' variety. I was unsure at first because the smaller size of button makes it appear that it is a plain back variety or 'name outside groove' which would have dated it a bit more recent. I had to recheck the backmark because the more recent variety has a slightly different shield as well. Your shield sweeps upwards at both ends and at the middle which would conflict with the more recent backmark that I mistakened it to have at first.
Now the cool part, Yours is an Infantry button which has the 'i' in the shield.
Shield with I : Infantry. Enlisted men, 1821-1854, officers 1821-1902.
Shield with A : Artillery. Enlisted men, 1821-1854, officers 1821-1902.
Shield with R : Riflemen. Enlisted men, 1821-1857, officers 1821-1902.
Shield with C : Calvary, 1855-1902 (officers only).
Shield with D : Dragoons (a light cavalry), 1836-1854 for enlisted men, officers 1836-1902.
Combining the Infantry shield(1821-1854) with the backmark being used between 1850-1865 only leaves 1850-1854 that this combination would have been used together.