Covert Charge/Cats/Chops

[Gareth Branwyn]

My wife (the jazz singer) came home from a gig the other night with a
question she wanted me to address to the list: A patron told her that in
one of the episodes of Ken Burn's Jazz, there is a sign that's visible
on the door of a club that says: "No Minimum or Covert Charges." This
person asked my wife if, despite the obvious, "cover charge" might come
from "covert charge." Seems unlikely, but I told her I'd ask.

Also, in the taped episode we saw last night, it was asserted that
Armstrong was "the first person to use" the terms "cats" and "chops."
Does anyone know anything about the etymology of these?