The only other bit I've been wondering, navy-wise, is when the garrison dixie-cap became regular use, over the combo cap.
I'm not sure which one you mean. The "dixie cup" is the enlisted sailor's white canvas hat, which goes back to the late 1800s. The garrison cap is the one that folds flat kind of like an envelope. They have been around since WW2: 1941 for aviators and 1943 for everyone else. In WW2 there were colors of garrison cap for every uniform: blue (black), khaki, green, gray and (rarely used) white. Now there are only khaki and blue.
I kind of liked officers in ST:09 wearing those covers outside at the Academy and in formal situations. It made it feel more real, like a real service branch.
Yeah, as a bald guy I've especially noticed the scarcity of headgear in earlier Trek!