An old thread with lots of great informed info., to which I can add only:
Other loudspeakers that incorporated the RTR ESL tweeter were the ESS Transtatic I (the first ESL driver I heard. I now own a pair of TS I’s) and the Fulton Model J (the ESR-6 cube, placed atop the Fulton Model 80 for the midrange, which sat upon a transmission-line loaded woofer enclosure). The Transtatic also used the famous KEF B139 oval woofer (in a 1/4-wave transmission-line enclosure), used in pairs by David Wilson in his original WAMM.