These are all ship covers to the U.S. bearing the 15cSC. Some have slogan marks.
This first slogan reads "Don't include money in your correspondence. Use the postal money order service or the declared value method."
This one is from the Rosecrucian Fellowship correspondence.
This slogan can be loosely translated as: "San Martin a key example amongst the freedom fighters of May" (I can't think of a direct word for 'prohombre', maybe 'helper of the common man'?).
This slogan reads "Dictate your telegrams by phone."
