Above I mentioned the ESS Transtatic I, introduced in around 1970. As I said, it consisted of a transmissionline enclosure (20" w, 16" d, 42" h) housing a single KEF B139 driver (also used by David Wilson in his original WAMM), a short transmissionline (the depth of the enclosure) behind a KEF B110 5" dynamic driver, and three RTR ESL tweeters (firing ahead and behind off an angled inner panel). I have a pair in my spare room.
Shortly before ESS introduced the AMT model (with the Heil driver), they offered what I believe they were calling the Super Quad. It was the same transmission line/B139 enclosure and RTR tweeters, with a Quad ESL for each speaker in place of the KEF B110, sitting side-by-side with the transmissionline enclosure. I saw a pair in the flesh, but didn’t hear them. If I remember correctly, ESS was selling them for $2,000/pr. (a fair amount of money in 1972).