Nice boxplate. Definitely pre-1865. It originally had iron loops to secure it to the cartridge box. The iron has rusted away on yours which they now mostly are. Congratulations on finding a great relic.
Man, what a NICE find! here's what yours looked like before the loops disintegrated. Very, very well done! Some unasked-for advice: Don't clean it much. A dry toothpick, your fingernail, and at most aggressive, the edge of a credit card. I will sometimes rub a little mink oil on the brass only after I'm satisfied with the cleaning, but some guys don't like to do that.