A small update regarding the search for these rare beasts...
It was uncertain who had made them, many thinking (assuming)
it was CDC; however, given the last of the three references
below, it appears it may have been made by Magnavox.
Still want one...
-dq
70mr02 Stifle, J. "A Plasma Display Terminal." CERL Report X-15, M
arch 1970; revised: June 1973. Also appears as "The PLATO IV Student
Terminal." Nov. 1974
74au01 Stifle, J. "The PLATO IV Terminal: Description of Operation."
Revised: Aug. 1974
97xx01 PLATO IV: Site Controller Communications Interface for the
PLATO IV Computer-based Education System. Magnavox, undated.