Dear Dan,
Thanks for your thoughtful remarks! Yes you are absolutely correct that Planck meant his resolution of the blackbody radiation "ultraviolet catastrophe" as a mathematical device, rather than as a physical quantum. It was Einstein who brilliantly suggested in his theory of the photoelectric effect that the quantum is a physical notion. Glad you like the mailbox metaphor! Interesting what you write about the distinction between encoding information and maintaining continuity within reality.
Many best wishes,
Paul