From the manufacturer, Clark Fobes:
My Eb extensions do not extend the range of the Eb clarinet. The extension fits between the bell and the end of the clarinet adding a properly sized and positioned tone hole to produce the E/B twelfth. Due to the much larger impedance of sound at the end of the clarinet the long B natural is often resistant and of a much different timbre than the throat tones. My extension greatly alleviates this problem - it also reduces the over sized twelfth interval. As a very happy coincidence the extension of the air column tends to even the timber of the Eb clarinet and produces a much more supple tone through all registers.