See
http://parkerguns.org/forums/showthread.php?t=17928
Imagine how much barrel weight had to be removed in the contouring/finishing process. A significant amount I suspect. I also suspect that barrel making in those days was not an exacting process. It was up to the skilled barrel finisher to make a set fit the parameters for a given gun. Some required quite a bit of metal removal and some not so much. The weight stamp probably helped with the selection of a set of barrels for a given application, minimizing the work required. Even back in those days time = money.