Bruce's answer is the most logical
, but stay tuned, as the cotter key is making a comeback in modern cars.
It seems that while cars no longer "high center" on deeply rutted roads, modern cars must contend with speed bumps, which results in "bottoming out" as bad as in the old days. The oil drain hole in both cases befowls with dirt & crud, for which problem the cotter key is employed to prevent.
Welcome back, Cotter!