The 20s, 40s, and 50s now all have the same driver.
"The new Fostex T20RPmkII and T40RPmkII now join the acclaimed T50RP in offering the true and accurate headphone monitoring crucial for recording studios, post houses, remote broadcast facilities, project studios and DJ’...
...This technology has been enhanced still further in a new RP Diaphragm (now standard across the range), which uses copper foil etched polyimide film to provide excellent resistance to high level input peaks of up to 3000mW (T50RP), ensuring an accurate sound regardless of the volume."
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2010#post_7663670