You can find more info about the old Sennheiser family on this great site (with other brands as well)
Thanks for that Japanese site link. It had a few tidbits of Sennheiser info I hadn't previously seen.
I only know that Sennheiser promoted Duofol (diaphragm with two diffrent thicknesses for stiffer dome and softer surrounding suspension) with the introduction of the HD590, in 1999. Quote from Sennheiser's press release: "Duofol transducer welds two Mylar films to make one rigid diaphragm. This is light and self damping, reducing resonances in the diaphragm itself and reducing flexing when driven hard."
I think Sennheiser successively implemented this technology in other models as well, which may indicate that there are early and late driver versions of the HD580 and HD600. And maybe the HD250.
Duofol is also mentioned in the marketing of the PX series and the HD555 and HD595.
I found John Borwick's December 1993 Gramophone review of the HD580 where he writes: "A new dual Mylar foil diaphragm is claimed to be self-damping and to have eliminated any distortion due to break-up modes." So this seems to be the first Sennheiser duofol design. His review is quite apt:
"The new transducer design at the heart of these headphones has produced a distinct refinement in performance. Listening to every conceivable type of music, I detected a special degree of resolution of detail and separation of the musical strands. I am always more comfortable with 'open-air' headphones than even the best totally enclosed types. Here I could sense the full space of the recording venue, presumably helped by the crossfeed externally between the two earpieces, and the diffuse field balance chosen by the designers. Stereo spread was close to the synthesized 'natural' sound stage we expect from spaced loudspeakers, instead of the exaggerated L/R divide that many headphones produce. The all-important centre image was still not quite located out front as I would like, but again it did not seem so buried in-head, or at the nape of my neck, as it often is.