You note immediately the dictator's initials come first in the business letter. Then, the secretary's or transcriber's initials follow in smaller letters. Although many managers do not have their own secretaries these days, this traditional way of placing reference initials still holds. As you can see, enclosure notations have as many varieties as there are people composing letters. You may abbreviate enclosure, or you may not.
Copies create a different situation in their notation.
That means someone enclosed a carbon copy or copy of something or the letter itself. Carbon paper disappeared many years ago. I doubt if you could find carbon paper, except inside some forms around the company. Postscripts mean exactly what they refer. You have forgotten to say something in the body of the business letter.