The Mk II was an "upgrade" of the original, classic 9mm Hi Power. The Mk II was intended for the military and police markets and finishes were normally either matte blue or parkerized. I've seen both. Some of the parkerizing was dark green while some runs were black. My first Mk II looked like it had been finished with a wood rasp! Others were much better in this regard.
Introduced in 1982, the Mk II was offered in but one caliber that I'm aware of, 9mm. It sported ambidextrous, extended thumb safeties and the classic Hi Power's checkered Walnut stocks were replaced with black, checkered nylon stocks with thumb rests. The magazine safety came along unchanged. The gun had a spur hammer and some were offered with lanyard rings. Frames were forged and at least in early runs of the gun, there was no internal firing pin safety. Some of the Mk II Hi Powers eventually did get the device as I've seen and shot Mk II's so equipped. The mainspring was the same 32-lbs that it is in current Mk III pistols and the recoil spring remained at 17-lbs, the factory standard. There is a small hole in the front of the slide, under the barrel and about where the center of the end of the recoil spring would be. Presumably this is to allow water to drain from a previously submerged pistol, something not uncommon to military handguns. The non-removable bushing extends out slightly more than on Mk III pistols. Classic Hi Powers made from the mid-70's until the Mk II was introduced did as well.
Perhaps the most noticeable change was the full-length rib on top of the slide. It is narrow and integral to the slide. The front sight is part of the rib and not removable. The rear sight is dovetailed in place and has a relatively wide notch for sight pictures in less than ideal light. These sights are usable at speed and most opine that they're an improvement over the classic Hi Power's fixed sights.
Until the introduction of the Mk II Hi Powers, the gun had been known to be picky in feeding some JHP rounds. The humped feed ramp was gone and replaced with a "throated" one similarly to what gunsmiths were doing here in the US to make Hi Powers feed reliable with other than ball. It's been my experience with several Mk II Hi Powers that they'll feed about any JHP of standard overall length or even close to it.
I am aware of no adjustable sighted versions of the Mk II nor any caliber other than 9x19mm.
FN manufactured the Mk II through 1987 are sometime near that. FN's records are more like "suggestions" rather than being as "exact" as other makers. It is possible that some variations of the Mk II exist, as its maker has been known to offer non-cataloged items to buyers having the interest and buying volume.
The Mk III Hi Power made its debut in 1988. It retained the extended, ambidextrous thumb safeties seen on the Mk II, but the barrel bushing was nearly flush with the front of the slide. Though it makes no practical difference, I like the looks of that better than the extended one. Gone was the full-length rib on the slide and both the front and rear sights are dovetailed to the slide. The hammer was the same spur type as on the '70's vintage Hi Powers as well as the Mk II. Oh, yes, we got to keep the damned magazine "safety" as well. Stocks were identical to the Mk II's. Mainsprings and recoil springs remained unchanged, but the hole in the front of the slide was not there on the Mk III. The lower rear of the ejection port is beefed up while the entire port is somewhat more "squared off" and similar to that of the 1911. Personally, I like not only the increased strength at the rear of the ejection port, but also think the Mk III pistol's ejection port change makes the gun look better.
Some Mk III pistols come with the lanyard ring, but most that I've seen do not. Finish will be either bright, polished blue covering the entire gun and is referred to as the "Standard." It costs more than the most often seen "Matte" finish, which is a baked-on epoxy finish over parkerizing and is corrosion proof. However, only the slide, sights, and frame have this finish. All other external parts do not.
Early production Mk III Hi Power's were produced with the traditional forged frame. It's my understanding that when FN tested the forty-caliber version of the Mk III the frame rails would warp or break at around 2500 rounds. For this reason, FN began manufacturing quality cast frames, which are harder. This solved the problem and allowed for the forged slides to be heat-treated a bit harder as well. The frames on the 9mm and .40 Hi Power's are the same and soon all Hi Powers were produced with the cast frame, probably to reduce manufacturing costs.
For those interested, how to easily spot a cast or forged frame pistol is here:
Though not commonly seen in the US, some Mk III pistols do not have the internal firing pin safety. However, the odds are that if you see or buy one, it will. While I don't think the device is necessary, I've encountered no problems because of it. There have been some reports of the slide cracking between the cutout in the slide for the firing pin safety's "paddle" to go through and block the pin, but this appears to be extremely isolated. Never the less, I'd prefer it if this device were not in the pistol.
More information on the internal firing pin safety and how to tell if your pistol has it, is here:
The Mk III is available in 9mm and .40 S&W. The frames are the same but the slide is slightly thicker on the forty-caliber version and it uses a 20-lb recoil spring rather than the 9mm's 17-lb. The mainsprings are the same. Mk III pistols in either caliber come with spur hammers.
There are other versions of the Mk III pistol beside the Standard. They are the Practical and the tangent-sighted Capitan.
It's my observation that the biggest controversy over the Mk III is the cast frame. Some detractors refer to this as "pot metal" or "cast iron" and roll their eyes while wringing shaking hands while saying that "quality" is gone from the Hi Power.
I shoot the Hi Power quite a lot and have done so long-term, as in 3 decades. I do believe that the earlier forged frame pistols had a "look" that is not matched, but I also believe that the cast frame pistols do hold up better for those shooting large amounts of hot-loaded 9mm in either handloaded form or +P. There is no choice for those choosing the forty-caliber. The forged frame pistols I own have not proven fragile by any means and I continue to shoot them, but prefer to pretty much stick with standard pressure loads.