For a reasonable price, a very clean-looking set of vintage high-impedance headphones was bought online. It needed only a light cleaning with a damp cloth, leather-wax on the headband and a polish (Brasso) to come up looking great again.
A quick, online search identified these as Neufeldt & Kuhnke headphones type Kt5a. Multiple sources give multiple years when it was made, but likely this was introduced in 1927 and in their range for 5 or 10 years.
Later variants have simplified sliders over the adjusting rods, but this specimen has the round clamps that are shown in what is probably the original 1927 illustration. These headphones then probably date from around 1930, give or take a few years.