I’m amazed that no one else knows it means children under 17 are not admitted on board.
Bones called it United Space Ship when he first met Khan in Space Seed, though I’ve always thought that United Star Ship sounded better. To me, space ship sounds like what someone would call a interstellar craft in the 1950s and 60s.
Yes, I am aware of when TOS originally aired.
It’s just something to put on your pizza cutter.

Well current airplanes have N-xxx numbers on their tails. So I assumed it was an outgrowth of that.
Didn’t even make it to post 3, I see.