An early attempt to beat spam filters?
I was recently looking at FIPS 74, the old standard from 1981 that defined how to use the DES encryption algorithm. Section 5 of this document is "USING DES TO MAP A CHARACTER SET ONTO ITSELF," which seems to indicate that the idea of format-preserving encryption has been interesting to people since at least 1981. FIPS 74 may also be one of the first attempts to bypass spam filters.
In particular, FIPS 74 (at least in the version that's on the NIST web site) section 5 is called "IMPLEMENTATlON OF THE A1GOR1THM," which has the letters "L" and "I" replaced with a "1." The first spam email was actually sent in 1978, so 1981 is definitely within the Age of Spam.
I don't recall email discussions of encryption algorithms being particularly overwhelming back in 1981, but could the title of section 5 of FIPS 74 have been an early attempt by NIST to sneak their DES standard past spam filters, sort of like people do when they try to sell you "V!@gra" today?