Right bifwynne, it was a cardboard tube as a "transmission-line" in the 2000A. I heard them at Sound Systems in Palo Alto, CA, hooked up to early SAE electronics, which were the hot amps until ARC made it to the West Coast. They also had the speaker ESS was making before Oscar Heil hooked up with them, the Transtatic I. It also had the RTR ESL tweeters (the static part of the model name) and a transmission-line (the trans part), but theirs a real one. In front of the line was a KEF B-139 woofer (the driver Dave Wilson used in pairs in his WAMM speaker), and then a KEF 5" midrange cone. I couldn’t afford their $1200/pr price at the time (or even the $600 for a pair of 2000A’a. The 1001’s were only $139 apiece), but I have a pair now!
That was in ’71, and by the next year they had the Infinity Servo-Statics and Magneplanar Tympani T-I’s in the same (large, obviously) room, running off ARC D-75 and D-51 amps. The guy who owned Sound Systems was kind of obnoxious, and I ended up buying a complete ARC/Tympani/Thorens/Decca/Revox system in ’72 from Walt Davies at Audio Arts in Livermore. Walt went on to develop and is making the Last Record Care product line, and is a great guy.