From all the ones I have seen, they're essentially the same thing. You have a metal mortise attached to the shank of the pipe that you then insert the metal tenon from the stem into. I have one at home that's a Nording Churchwarden. I believe that the terms basically describe the same style of fitting, but every brand is different. I think Peterson calls there's a spigot.
Both are 'army mounts', the difference is a spigot has a metal tenon. They are both designed to be pulled apart while hot.
I love army mounts, specifically silver spigots. Also, all Peterson System pipes are army mounts.
I have a small collection of silver spigots, The one on the right is gold mounted.
I think spigot is a fit with metal on both stem and shank. Army mount only has metal on the shank. I have seen Dunhills and Castellos with acrylic army mounts as well, so I guess it doesn't have to be metal.
Okay, this is the part I never understood about the whole "reinforced with a bullet casing" thing. A casing has one end closed, where the primer is located. So, how does a guy in a trench take the back part of a bullet casing off? You need a hole that goes all the way through, right?
So army mount is reinforced shank with friction fit stem, spigot is metal on metal. Not always metal but reinforced. The "psuedo" is a traditional tenon without friction fit. The story is likely BS, as many of these stories are. But pipes are often made with a casing anyway and it is awesome.