The model 1 Eminent Technology is listed for 1983 in their product history. The ET arms have the arm tube attached to a center rod that is pressurized from outside by air coming from the stationary air block, like Walker and Maplenoll. The Bergmann, CS Port, Forsell ,VYGER and Holbo have the armtube attached to a guide that is pressurized from the inside by a stationary perforated center rod.
The Forsell model seems to be more popular these days, less required pressure and less moving mass maybe ?