This week, I thought I would do a "retro" measurement of and listen to a headphone from the days of the Cold War. ;-)
Above, you see the 600Ω impedance Beyerdynamic DT 990 (forefather of the current DT 990 Pro, ~US$150), a pair of headphones lent to me by my buddy linnrd that he has had in his archives of audio equipment for quite awhile. As you can see on the box, these were made in W. Germany and purchased back in 1991, a couple of years after the fall of the Berlin Wall; either in Düsseldorf or nearby Essen I was told. The history of these headphones began back in 1981.
For headphones of this vintage (>30 years old), they are in great shape although in the image above, notice that I had replaced what used to be a thin layer of disintegrating black foam inside the earpiece with some bluish 1.5mm felt which I had used in my Dekoni Blue mod last year. So, not exactly a "stock" unit but pretty close.