I assume you mean a MK IV not a MK II....since there are no separate patterns for the MK II. First for the obvious, if we are talking the one at the front of this post, it is a Pattern "A", with the dead give away being the short knoxform visible in the block picture. Also it should have a plain front sight and not a ramped one like the "B" and "C" do. Now back to telling the difference. While there are a number of subtle differences, I'll stay with the ones you can spot externally from 5 feet away.
The "A" has a new receiver with the "IV" centered under the lock viewers mark. The "A" while a rebuild from a Enfield Martini, the original EM rifles had a number of features attached to the receiver that made using a new one just easier. The knoxform is very short and as stated, it has a plain front sight and not a ramped one.
The "B" reuses the later model EM receiver. While you'd think that the date on the receiver would be a help, both EM and MK IV rifles had receivers with dates overlapping MK IV production. With the overlap, the date isn't definitive.....but the MK "IV" is not centered under the lockviewer's mark. Since these were originally EM MK I rifles the "I" was centered under the lock viewer's mark. When converted to MH MK IV rifles, a "V" was added to make the "IV" thus making the mark offset to the right. The Pattern "B" has a slightly shorter (by one thread width) knoxform and barrel than the Pattern "C", but unless side by side you won't notice. You will notice it has a ramped front sight.
The Pattern "C" was supposedly assembled from all new parts, including a new receiver that should have the "IV" centered under the lockviewer's mark. It also has the longest knoxform and has the ramped sights.
Now some will say that while the "A" and "B" reused the EM buttstocks which had the original retaining cup for the short lever filled, the "C" used new buttstocks that don't have that hole filled. I've had a number of MK IV rifles go through my hands of all patterns and have gone through pallets of them at AC. I have never seen a buttstock without the filled hole....not a single one. While I'm quite willing to grant they exist, I can only conclude from my observations that while the "C" were all new parts, that many of the new buttstocks were originally made and placed in stores as spares for the EM and then were used in assembling many if not most Pattern "C" rifles. If it was otherwise, you'd think that we'd routinely see buttstocks without the fill......but we don't.
Now as I caution folks routinely that point out discrepancies between their rifle and a given pattern, all of these patterns were in production at the same time in the same factory by the same workmen.....and I'm sure EM parts were ruined in the conversion with new parts substituted....as happens with all wholesale conversions. As such "brand new" MK IV rifles likely had deviations from the accepted standards. Also almost all of these have been through one or more rebuilds where "parts is parts" govern what gets assembled with what. The most pertinent difference is that the "A" and "B" reused the earlier EM barrels which were rebored, rerifled and rechambered for the 577/450, leading to slightly different tolerances. Aside from that, the only other practical difference is that the Pattern "A" will accept the earlier socket bayonet as well as the later blade(sword) bayonets, while the "B" and "C" owing to their ramped front sights won't accept a socket bayonet.
FWIW I prefer the Pattern "A" as shown at the front of this post. I can't give you a good reason why beyond they are the least common and still take the less expensive socket bayonet....which I've got quite a few of at this point. My regular shooter is a pattern "B", likely because it was my first MK IV and is accurate enough that I haven't had an itch to work with the others too much. From my point of view, the MK IV regardless of pattern is the best for shooting of the 577/450 MH family.
As to the question on using a muzzle protector, I see no need for one using a coated rod. Others would disagree. To each their own. Using one certainly doesn't hurt anything and if it lets someone sleep easier at night, so be it.