Possibly the trapdoor had 4 variations between the end of the civil war and 1871 1865(.58 rimfire) 1866(.50 caliber conversion) 1868 .50 caliber with a new barrel and 1870 the last of the .50’s.In 1871 it would have to be an "Allen conversion" from musket to trap door would it not?
Would be very interesting to find out where/who that brand is registered too.Horse is described as an Indian Cayuse