Proper nouns should name specific people, places or things – “specific” being the key word here. In some cases specificity is immediately apparent, e.g. a hypothesis or an example of this type of specificity (think: how are some specific products represented, for example)? “Singapore Airlines”(after all, there is only one airline with that name!).

In some cases, there is a gray area and I would say that fame plays a role in determining specificity. To use your example, one would refer to the Guantanamo letters if they don’t contain any content that is particularly newsworthy. If, on the other hand, these letters happen to disclose some salient information that produced a political scandal, then these letters would rapidly garner attention and become intimately connected to this political scandal in the minds of people. In such a link, the letters will be infused with a specific quality that would warrant the use of a proper noun, i.e., the form “Spy” or “Sword”. Guantanamo Letters.

What will happen if some of the common nouns become real with time? Is it “ok” to call a verb a proper noun?