Just my tuppence worth:
The F-S dagger was excellent for it's intended role - it's perfectly balanced, slim, light, easily concealable, and the idea was that the commando would sneak up behind the sentry, place his hand other the mouth and nose of the sentry, and strike at either the kidneys or side of the neck, thus taking the sentry out silently. It was used thousands of times in WWII and later conflicts for just this purpose.
At the time, american soldiers did not have a standard issue knive. A bayonet, but no knives. when they came over to England and trained with our commandos, they loved the knife. But Americans being Americans, they tried to use it to open beer bottles, dig holes, carve thier name into trees, as a lever bar, and would insist on trying to "take out" sentries by trying to slip the blade between the ribs into the heart¹.
Thus the Applegate-Fairburn was born. a redesigned knife to make it stand up slightly better to the average (not all, but the majority) American soldier's incorrect uses of the blade.
Soon after, the US forces gave up entirely on this design and started using large, flat, heavy, unbalanced clip-point blades (see: kabar) this was perfect for opening bottles, digging holes, and beating things. For actual killing however, it's not so great. As a slicing/hacking tool, it works. Brutal but effective, a true multi-purpose knife and the first real "ultilitarian" knife, but a specially-designed killing knife it is not.
One interesting point of note - the British Army still issues the F-S knife to certain units for the specific role it was originally designed for. It also issues a seperate knife for "utility" (see: "emergency knife"), a seperate knife for beating things, use as a pry bar, tent peg etc (see: "survival knife"), and a seperate knife for hacking through the jungle (see: "machete/panger"(the latter being the nickname), one very unlike most contempory designs but I won't go into that as it'd be dragging this thread off-topic). And everyone in the Amry is issued with a folding knife. Made of sheffield stainless (I have no idea of the exact composition, I'm no metallurgist) which has one "sheeps foot" blade, and one bottle opener (with the flat screwdriver bit at the end as copied on millions of SAKs). Naval versions incorporate a marlin spike, and I've no idea about the RAF version. As you can see, instead of trying to solve every problem with a single blade as the US Army tried, our army just use specific tools for specific jobs. and no mention of the British army's knoves would be complete without the mention of the much-lied about Kukri. It's not a blade for ritual sacrifice of *any* animal, it's not a fighting knife, it's not a machete, it's the Nepalese equivalent of the bowie knife - does a little bit of everything. There is also no compulsion to draw blood with the blade every time it's taken out of it's sheath. Given it's one of the world's oldest "utility" knives, there would be no Gurkas left if they had to bleed themselves or thier companions every time they wanted to use the knife for it's many thousands of uses.
As for manufacturers, they vary. The 1st version was indeed made by Wilkinson Sword. This had a slightly slimmer blade than later version, and a distinctive "s" shaped guard. The 2nd version had a thicker blade and handle, and adopted the straight guard most seen. the 3rd version was very similar to the second, it was introduced in 1943, and remains the current type. Different makers and markings are abundant, but basically if it doesn't have a "crows foot" (properly called a wide arrow mark, the official stamp of the MoD) and a makers name stamped into the back of the guard, it's a replica. If the maker'sname is anywhere else, it's a fake. If there's no crows foot, it's a fake. If it has numbers on the handle near the guard and one of the bottom of the pommel, it's either a fake or a rare 2nd version. If it looks like a first version, it's almost certainly a fake/replica, as very few of these have survived and most are in museums and private collections. If it has a badge of any sort attached to the guard (as seen on most F-S style knives on eBay), it's a fake.
Please not this is all taken from my often-failing memory, anyone who'd like to correct me on any point(s) is welcome to.
Danny
¹One thing many people fail to realise is that the best route to the heart truly is through the stomach. If you truly wish to pierce his/her heart, first gut your victim and then go up through the intestines under the sturnum. You only attempt to crack open the sternum/ribcage if you want the victim to live afterwards (see: operations involving the heart - transplants, bypasses, etc)
==edit==
Added a small point to the fifth paragraph that I've just remembered. Also please ignore typos, it's 07:47, I've been awake since 02:00 yesterday, have worked a 12 hour shift, and have consumed a couple beers.