
We know Mecklin did this because there's a certificate in his papers to that effect, reading "KNOW ALL MEN BY THESE PRESENTS THAT John Martin Mecklin borne on the wings of the PH-LDG, a Flying Dutchman of KLM, Royal Dutch Airlines, has had the distinction of crossing the Equator." The certificate is illustrated with Aeolus, the ruler of winds in Greek mythology, sending an airplane and a ship on their way.
Documented as early as the 17th century, line-crossing ceremonies are a variety of folk practice surrounding the first time someone, typically a sailor, crosses the equator. They can range from entirely anodyne to outright hazing and assault. This is a funny commercial example from the '50s -- one imagines there couldn't have been much of a ceremony onboard the plane itself, but the airline clearly found it worthwhile to produce and distribute personalized certificates for its eligible passengers.
To see this and other travel souvenirs, request ML-28 Box 2 Folder 10.