"mind your p's and q's" comes from "Pints and Quarts"; however, I'm not sure the exact context, whether it was multiple pints and multiple quarts, or things that belonged to the pints and to the quarts.

I would guess that "do's" is a throwback to post medieval times (thinks c16 / c17) where the grammar rules and spelling was somewhat more random.

What do you think of "C.D.s"?