This is a common notation. Think about our letters. Remember seeing any marked Par Avion and Via Airmail? I have, and it is simply another example of a little French being used as a part of another language. It goes back to the pre-WWII days when French was the "international language" and the "language of diplomacy." There were numerous such examples 50 years ago. Many fewer today, but it still shows up from time to time. Vincent Falter