I believe that Lefever & Ellis were partnered between 1862 & 1867 & were making muzzle loaders. I have seen one gun from this era, an over/under combo of 16ga/.40cal having a righthand mule-ear percussion lock with two hammers, one trigger as I recall. I presume it dropped which ever hammer was cocked. Agree with 8-Bore, it sounds like later Lefever/Dangerfield design. Pictures would definately be great.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra