The inclusion relations of the countable models of set theory are all isomorphic

The structures $⟨M,{\subseteq }^M⟩$ arising as the inclusion relation of a countable model of sufficient set theory $\langle M,\in^M\rangle$, whether well-founded or not, are all isomorphic. These structures $⟨M,{\subseteq }^M⟩$ are exactly the countable saturated models of the theory of set-theoretic mereology: an unbounded atomic relatively complemented distributive lattice. A very weak set theory suffices, even finite set theory, provided that one excludes the $\omega$-standard models with no infinite sets and the $\omega$-standard models of set theory with an amorphous set. Analogous results hold also for class theories such as G"odel-Bernays set theory and Kelley-Morse set theory.