Era
The 1960s brought the high-fidelity headphone into homes and studios. Stax sold its first electrostatic in 1960 and the SR-3 in 1968, Koss followed its stereo headphone with the Pro/4 in 1962, Beyerdynamic dates its long-running DT 100 to 1965, and Sennheiser's HD 414 of 1968 showed that an open headphone could sit lightly on the ear. Many were made to plug into a receiver's speaker outputs or a DIN socket, so expect odd plugs and high impedances. Foam from this era has almost always perished, so look at the pads first.
3 models we cover.

Sennheiser
Open-back$160–$200 typical ask
6 on eBay
Beyerdynamic
Closed-back$120–$360 typical ask
12 on eBay
Stax
Electrostatic$180–$210 typical ask
4 on eBay