Check out this site. Great info.
https://www.trapdoorcollector.com//Al has literally written the book on Trapdoors. The Bufington sight is very common and very likely the gun was sent in for service at some point and the Buffington was put on it.
The 1859 and 1863 Sharps was converted to 50-70 following the Civil War, but yours may have been re-done later on by a gunsmith. I have one that I carry a couple of days a year in deer season. Mine was in a 20 gun serial number block issued to the 10th Ill. Vol. Cav. in 1863
If you can find it, SR4759 is a great powder for TD loads. I found it much cleaner burning than 5744. I stay away from black powder as I just hate cleaning the cases.
