The image is based on the information in the similar diagram in Counterfactuals and Comparative Possibility, page 28 of volume 2 of Lewis's Philosophical Papers. A different depiction of the information/diagram is at Lewis, David (1973). Counterfactuals. Blackwell, page 131. The diagram was drawn in SVG from scratch, to avoid infringement.

Lewis's systems are all-and-only the different (nonredundant) combinations of these axioms, where the letter V is just a placeholder and does not signify an axiom.

N   □⊤
T   □A ⊃ A
W   AB ⊃ ◇A  &  A ≼ B
C   AB ⊃ A < B
L   (no further axiom, or some tautology)
S   A □→ C  ∨  A □→ ¬C
U   □A ⊃ □□A  and  ◇A ⊃ □◇A
A   A ≼ B ⊃ □(A ≼ B)  and  A < B ⊃ □(A < B)

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