Ah, now you're making me feel really old for knowing the answer to this. Sigh. It stands for "carbon copy." It comes from back in the days when you could put a few sheets of paper in a typewriter, with thin sheets of tissue paper coated on one side with carbon (or later other less messy material) in between the sheets of typing paper. And when the keys of the typewriter hit the paper in front, creating the orignal letter, they also created a "carbon copy" at the same time. The "cc:" indicated what was done with the carbon copies. Now it simply designates who is going to recive photocopies, or if it's from my office - usually .pdf copies, of the letter.

The other common "code" at the end of a letter, would be three letters in upper case, separated by a colon, and then two letters in lower case. Such as: TBM:mt Those designate who authored the letter, in the example Timothy B. McCormick, and the typist, in the example my old secretary Missy Thomas. I never really understood the point of that one, since the author was obvious and it never seemed to matter who typed it. Now a days, of course, the author is equally obvious, and the typist is embedded in the metadata of the document file. So many people, myself included, have stopped using that code.

Cc as now used just tells you who is getting copies of the letter.

The purpose of the typist's initials was that offices used to have typing pools, big rooms where all of the typists would sit all day cranking out the documents. If someone wasn't busy, the boss would give the work to her. By using the initials, one could go back to the original typist to make a change.