I just attended your talk at Apple, and I thought I'd try to get this to you before you get home.
The Dead Medium in question is the CYROGRAPH. It was a form of authentication for duplicate documents used in the Middle Ages. The document was written in duplicate on a piece of vellum (or parchment); the copies were cut apart and retained by two different parties. Sometimes the cut was deliberately irregular in order to make spurious matches unlikely. In addition, lettering would be placed where the cut was to be made so that both the shape of the cut and the lettering would have to match in order to authenticate the copies.
References (from Library of Congress online catalog):
92-131963: Brown, Michelle. A guide to western historical scripts : from antiquity to 1600 / London : British Library, 1990. 138 p. : ill. ; 29 cm. LC CALL NUMBER: Z114 .B87 1990
92-160830: Brown, Michelle. Anglo-Saxon manuscripts / Toronto ;
Buffalo : University of Toronto Press, 1991. 80 p. : ill. (some col.) ; 24 cm. LC CALL NUMBER: Z8.G72 E53 1991
-- Dan Rabin (firstname.lastname@example.org)