The B139 is also the woofer in the ESS Transtatic, and is the reason Oscar Heil ended up partnering with ESS for his new driver. He came into the ESS dealer in San Jose (where I heard the Transtatic, and was lusting for), looking for info on how to get some of the woofers, which he wanted to use with the loudspeaker he was working on. The shop put him in touch with ESS, and the rest is history. They didn’t incorporate the B139 in the ESS/Heil speaker, unfortunately (it cost to much to allow the Heil to retail for what they wanted, I believe).
I bought a pair of Transtatics in 1982, and the B139 in one of them had been replaced with a generic knock-off. I called ESS, and got the last remaining B139 they had in stock! Still have 'em.