Could we have a better picture of the marking? The gun is a saw-handle type (so called because the grip shape was the same as handsaws of the period) pistol and appears to be all metal, probably brass.
I thought it was brass and it is definately all metal. I have attached a couple more pictures of the markings. My dad had given this to me 35 years ago in a box with some powder flask and other things. Just thought I would try to get some history on it for my children.
Dion Brothers, New Orleans
As stated above, it is a saw-handle pistol, relatively common around the middle of the 19th Century here in the USA. Very nice workmanship.