Back to you Alan,
My grandmother, born in England and grew up in the Isle of Man, always used to remind my brother and I to "mind our P's and Q's" whenever we were tending to not do our homework after school. I haven't any idea of where it came from but my dear wife, who reads everything she can find, may know. I'll test her.
Re Mark Twain, I know where the name came from but will let you enjoy the fun of finding it. Let me know when you give up. Several years ago, an actor by the name of Hal Holbrook, made a TV presentation of the old and recorded sayings by Mark Twain. I saw it every time it was repeated over several years and it was incredible.
Anyway, it was on that presentation that I found out where his pseudoname came from.
You are close!
George
