I have a 4 inch barrel Colt revolver whose barrel is marked TROOPER MK III, but which is attached to what must be a Metropolitan frame (Serial J61xxx). The barrel is clearly marked for .357 magnum, but this is a fixed sight revolver and the frame was clearly not ever wearing adjustable sight (has a normal, fixed sight channel). The top of the barrel is nicely milled flat, except where it curves down from the front sight. It doesn't look like a gunsmith job, but could very well be. Or a factory mod. Can anyone help me with this?