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All answers having "Pen pals?" as clue:

4 Character Answers:

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CONS

HOGS

PIGS

SOWS



5 Character Answers:

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BOARS

SWINE



6 Character Answers:

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FELONS



7 Character Answers:

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INMATES

OINKERS



9 Character Answers:

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CELLMATES

PRISONERS



11 Character Answers:

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