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The pattern that the line-ending words follow is often explained if the numbers 1 to 6 are allowed to stand for the end-words of the first stanza.

Each successive stanza takes its pattern based upon a bottom-up pairing of the lines of the preceding stanza ( i. e., last and first, then second-from-last and second, then third-from-last and third ).

Given that the pattern for the first stanza is 123456, this produces 615243 in the second stanza.