"P lip" refers to the lip part of the bit in some Peterson pipes. In the Peterson Lip, the hole points upwards from the bit, towards your palate, as opposed to outwards, towards your tongue. It's supposed to spare your tongue from tongue bite. Opinions differ about its efficacy.
A fish-tail lip flares out like a fish's tail, thus giving one more grip surface for his teeth. It's supposed to spare strain on the teeth.
If you put your pipe in your trousers' pocket and then mount a horse (or bike), you'll break it. So an army mount is where the shank has a reinforced end, usually of silver, and the whole bit, without tennon, fits into it. It's supposed to make it easier to separate the pipe while you carry it in your pocket, so it won't break, and easier to push the bit back into the shank, even with one hand, the moment you want a smoke.
Where both the shank and the bit are reinforced with silver, it's now usually called a spigot, although my grandfather called this a "cavalry bit".
