The Lee version/tool is very economical and works well for expanding or flaring the mouth of the case. It does not go very far down into the neck of a bottlenecked case or a straight case. Some handloaders have achieved the same effect by using a set of closed needlenose pliers or a tapered punch or a tapered nailset. The Lyman "M" die is a somewhat different arrangement. It is a straight cylinder plug with a slight bevel at the bottom to aid entry into the case mouth. At the upper end of the shank/plug there is a slightly larger in diameter-step. The cylinder/plug expands the entire length of the neck of a bottleneck case unless you adjust it to do less. It will also expand the upper part of a straight case the same way. If you have it adjusted to go to its maximum length then the little step enters the case mouth and flares it a bit larger to prevent lead shaving and to ease bullet entry into the case mouth. It is a very effective aid to proper loading of cast bullets. The "plug" or short section/cylinder with the step is unscrewable or detachable from the main cylinder that is within the die itself. So the parts are interchangeable. As already pointed out most of the dies vary in the length of the body being either long or short. In a couple of early editions of "The ABC's of Reloading", Dean Grennell, gun writer extraordinaire, devoted a chapter complete with photos and clear instructions to making plugs with steps of different diameters to fit various calibers,,,that fitted the bottom end of the center portions of the Lyman M dies. Its not rocket science and having a lathe, a drill press or a machinist friend really helps. Grennell also made many of his plugs come to a conical point in order to eliminate entirely the possibility of the mouth of the case catching on the bottom of the plug. Its worth checking out if you are a user of M dies. Good shooting. JC