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.
Functionally the headphones are still fine, no breaks. The permanent magnets have lost most of their strength though; the membrane did not even need to be slid off, but could be simply picked-up. (Generally, the membrane should be slid sideways out of the magnetic pull so as to avoid any bending or damage.) With weak magnets the quality and strength of the sound is probably not what it once was, but they do work.
One change to the original item was however made - the connector plug was removed.
It probably is original, also dating to around 1930, but that's a 220V mains plug. It really is a mains plug, rated 6A and 250V with pins at 19mm pitch and made of bakelite.
Despite taking away from the object's historical integrity, this plug was removed. Because everything is screwed, it's even a reversible procedure with nothing having to be cut. But having a plug that fits regular 220/230V 50Hz power sockets attached to headphones with probably dodgy insulation is just a bad idea from safety perspective.
Even when new, back around 1930, this must have been a bad idea surely. Around that time the power socket installed in buildings started to settle on the 4mm pins at 19mm pitch. Even then it would've been possible to use another type of socket and plug to get a two-conductor connection.
The original crimped wire-ends were instead clamped in the terminals of a cobbled-up audio-jack converter. Much safer.
And this also makes these ~90 year old headphones usable today on any device with a regular 3.5mm audio phone connection.