I found this clip from air facts: It looks like pax for passengers goes back at least to the 1940s in the airline industry. The magazine for pilots, 1946 (check):
大家好,这里是pax britannica submod : liberte ou la mort制作组,在紧张的工作以后,我们终于能够将lom2以及《抗战到底》dlc适配后的内容正式公布,而以后,我们将聚焦于英雄交. Here's the full text of the reference:.