On the Wikipedia page - http://en.wikipedia.org/wiki/Saga_Ruby - it refers to her as being RMS Caronia - this isn't true is it?
I can't see how, as she sailed in the fleet as a cruise ship only not as a liner. Wikipedia is only as good as its contributors so mistakes do frequently pop up. I remember discussions occurring in the Pursers Office regarding QE2 carrying the mail in certain circumstances though - can't prove that though, totally hearsay, but would seem possible with British Airways strikes we may have taken the Mails across the Atlantic.
What I do remember is that Cunard never used the prefixes RMS, SS, MS or MV on any literature, letters or memorandum that I ever saw when referring to the existing vessels of the fleet. It seems to be the enthusiasts who insist on using them.