[Reece]

This is a very nice-looking high-impedance speaker intended for use with radios built in the 1920's. The third "wire" is not a wire but just a continuation of the cloth covering over the cord. Sometimes people used this to tie to a point on the radio as a strain relief so that the wire-to-pin connections wouldn't be broken.

You can test the speaker with a flashlight battery or a 9-volt transistor battery. Touch the phone tips to the battery terminals briefly and you should hear a click in the speaker IF the wire still has continuity, and IF there is still continuity in the driver. The adjustment is probably to locate the driver armature in the magnetic gap once the speaker is working.

The driver probably has an impedance of 1000 or 2000 ohms. Most speakers today are 4 to 8 ohms. If you get clicks with the battery test, you could test it (even though the impedance match would be off) by connecting the phone tips via a patch cord (alligator clips on one end, phone plug on the other) to a transistor radio, cassette, or CD player. I am suggesting a low power device like these so that you don't hurt the old speaker. I wouldn't suggest connecting to the speaker connections of a stereo amplifier. This speaker can't take much power. The sound is not going to be very impressive.

If you're not into 1920's radios, this speaker would not be of use to you, and you might list it for sale in the usual place(s), where there would probably be interest, even if not working (can be repaired).

Reece