The Sizlak Curve

A NSA FOIA processing file cites "The Sizlak Curve" as a source searched by the Cryptography Office for documents related to the British Non-Secret Encryption, precursor to publc key cryptography. Information on what "The Sizlak Curve" means is welcomed. NSA says the term is unclassified. It could be disinfo or a joke. An image of the citation: http://cryptome.org/sislak-curve.jpg The processing file from which the image is taken: http://cryptome.org/nsa-nse-061208.zip (142pp, 4MB) Forwarding this inquiry to crypto lists and fora would be appreciated.