Since it doesn't seem to have been addressed yet, I thought I'd pass along the content of the original release form which was signed by S&S (courtesy of Harlan Ellison's essay, "Did YOUR Mother Throw YOURS Out?" reprinted in EDGEWORKS 3). Hope it provides some illumination.
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"I, the undersigned, am an artist or author and have performed work for strip entitled 'SUPERMAN.'
In consideration of $130.00 agreed to be paid to me by you, I hereby sell and transfer such work and strip, all good will attached thereto, and exclusive right to use the characters and story, continuity and title of strip contained therein, to you and your assigns to have and hold forever and to be your exclusive property and I agree not to employ said characters or said story in any other strips or sell any like strip or story containing the same characters by their names contained therein or under any other names at any time hereafter to any other person, firm or corporation, or permit the use thereof by said other parties without obtaining your written consent therefor. The intent hereof is to give you exclusive right to use and acknowledge that you own said characters or story and the use thereof, exclusively. I have received the above sum of money."
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- Frank