Hi Feltwad,
Thank you for the photos. However, I disagree that they represent La Chaumette's design. In fact, the screw-cap breech you show predates him by a long time. In his book on English guns and rifles, John Nigel George wrote that there is evidence of screw-cap breechloading rifles used during your civil war. Of course, I am assuming George was accurate because I have never seen a screw-cap gun from that time period. Mike Brooks briefly owned and showcased a beautiful English example that was made during the latter half of the 17th century. La Chaumette patented (but may not have invented) a screw plug breech that went completely through the barrel, like the later Ferguson system.
Again thank you for the photos. Do you know the maker of the rifle? I always appreciate your contributions on this forum and enjoy the photos you provide. I learn a lot from them.
dave