Using the words "Quality pen", one letter per year, followed by a quarter marker, allowing for dating within a ten year span.
So far the earliest date coded pens found have had the code NL for 1979, and all of these pens were made in the US.
Nice to know that if the am call comes, there might not be anyone home. Source ABC evening news of June 22, 2009, I dont know where ABC got the news feed, but I could check if you need to know........................................
Update: Okay, so Sanford apparently called his office after the AP report.
Hence a "47" marking on a 1930's pen indicate that the pen was produced in the fourth quarter of 1937, not 1947, which is a common misconception.