Yes, the other quotes are "modern system" and "secret modern code".

This fits in with "Scheidt's underlying goal" ... the sculpture as "in some sense, a history of cryptography" (Gillogly, Washington Post, July 1999).

(December 1989 - in the letter you quote)

The right side is a text that can be partly deciphered by using 
the table and partly by using a _potentially challenging 
encoding system_. The text, written in collaboration with a 
prominent fiction writer, is revealed only after the code is
deciphered.

(January 1990 - quoted e.g. in #15893 here, through FOIA of NSA presentation quoting Washington Post)

Anyone who knows a coding system called the Vigenere Tableau, invented in 1586 by French diplomat Blaise de Vigenere, will be able to decipher one-half of the phrase. The other half will be encoded in a modern system created for the project by an expert cryptographer, whom Sanborn would not identify.

(March 1991, "Art in America" ... not quoted here yet, but not really adding much)

The work's opening section is written in Morse code; subsequent sections are written in Vigenere Tableau, a cipher invented by a 16th-century French diplomat, and in a secret modern code.