In theory, according to the contract, the rights revert automatically without requiring a reversion letter ("all rights hereunder shall immediately revert to the Author"). In practice, however, a new publisher will want proof of the rights reversion. A rights reversion letter is the usual proof, but the new publisher can choose whether it will accept alternative documention if AMP can't or won't provide the letter.
Usual disclaimers (this is not legal advice, blah blah blah) apply.
Usual disclaimers (this is not legal advice, blah blah blah) apply.
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