I was watching the movie Sand Pebbles and noticed some sailors had their insignia on the right arm. I Googled it and came up with what I copied below. I would appreciate it if someone could translate it to civilianese and perhaps share some insight.
Beginning in 1913, Sailors in the seaman branch were required to wear their insignia on the right arm, while all other ratings wore their insignia on the left. This practice gave rise to the term "right arm rate," signifying a member of the seaman branch.
Thank you.