"Cover sheets" are pieces of paper that you put on top of a folder to indicate its classification status. They presumably exist to allow people with clearances to know, at a glance, what the highest level of classification of documents in a given folder is, without actually opening up the folder. They're a reasonably simple part of the practice of secrecy: they help you quickly determine how secret something is supposed to be.
Today these are rather standardized. Below is one for SECRET/RESTRICTED DATA that I came across in the National Archives, denoting a variety of different classification statuses. My guess, based on the various form numbers assembled on them, is that this model form dates from the late 1980s, but it might be more recent than that.
These are terribly dull, but very effective. They scream out to you pretty vividly how fearfully they should be regarded, they are pretty unambiguous in their meaning, and they're on attractive card stock. They tell you both what it is, and what you should be doing (or not doing) to the contents. These things are littered throughout the files of the Atomic Energy Commission.
Let's look at some earlier models, though.
Here is one that was developed in late 1949 but used through at least the early 1950s:
This one is much more visually interesting. The font choice is very early Cold War, is it not? Not quite as scary as the 1980s version. I don't think you could get away with using a Lithos-like font for something this serious today, but there's something about its apparent frivolity that strikes me as even more disturbing than the sea of Helvetica that is the coversheet from the 1980s. There's a casual quality to its warnings. "Hey there, chum. Better treat this document correctly, or we'll send you to the electric chair!"
The cover sheets didn't always look so slick, though. When the "Top Secret" classification category was formally created in 1944, the classification bureaucracy was a lot less formalized.
General Leslie Groves, military head of the Manhattan Project, used very idiosyncratic looking "Top Secret" cover sheets:
This one is from May 1945, attached to the "Rules for Public Release" that governed what was allowed to be published in the final version of the Smyth Report. As you can see, these cover sheets also served as logs as to whom the documents had been given to, when people had requested it (in the above document, Groves himself checked it out a few times), how many copies were made, and other "metadata."
That loopy script at the top was common to all of Groves' "Top Secret" sheets. I can't help but wonder who came up with it. I almost want there to be little hearts above any dotted i's — it has an especially whimsical appearance. A few Lisa Frank stickers and it could be in a high schooler's binder.
Looking at the full spread of these, it's hard not to take away the conclusion that in their quest for appearing serious about secrecy, the U.S. government has gotten increasingly dull in its graphics design. This is, I am fairly sure, one of the least significant criticisms of secrecy ever to be leveled, but hey, you heard it here first...