[RJay]

It is a turn off barrel, designed to be loaded with the barrel removed. The four splines are for a " key " or wrench to fit inside the barrel to apply the force needed. Once removed, there is a cavity in the receiver that, first the powder, then the ball is place on top and the barrel reinstalled. Your gun looks to be in very good condition and well cared for. I don't think rust would be a factor, however after a 150 years the treads may be very tight. If you have a tool made to remove the barell I would remove the grips and soak the area in a penetrant first. The firearm is in wonderfull condition, some one took very good care of it. Good luck.