The back action lock has been in use since 1830 or so. The cheek piece is a style usually used on full stocks but not always. It could be closer to the flintlock era than 1840 but I don't think so. IMHO I don't believe that patch box came on it and the horse and deer are later add ons but the clipped corner rectangle on the cheek piece is original. The breech is definitely hooked and it is possible there are markings on the underside of the barrel. I'm going with Jim and say it's from the 1850ish -1860ish time period. I don't believe it's older than that but I've been wrong before.