AKG K-100 is similar
I bought mine in 1974 and they still work fine.
They needed a cable change long time ago,
and the plastic headband was replaced recently
with a hand made leather one.
Bass is lacking somewhat on these, too,
presumably due to insufficient sealing.
And there is no diffuse-field equalization either.
That makes them sound quite different to today's
headphones, but the difference diminishes after
applying the diffuse field equalization at the source.
I have no technical data for AKG K-100 (apart from
impedance being 2 x 600 Ohm) and would be very
grateful if someone could quote basic parameters.