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It could be a coil to cancel out hum, however another (more likely) possibility is that the audio output transformer may also mounted on the speaker. If this is the case then the B+ is connected to one side of the primary of the audio output transformer and the other side goes to the plate(s) of the audio output tube(s). Then the secondary of the transformer reduces the impedance to the voice coil somewhere between 2 to 8 Ohms. Some detailed pictures would help a lot. Ohm-ing it out can tell you a lot also. Also another thing you can do is gently push on the center of the speaker and if you feel the voice coil rubbing, that's not a good thing. It should move freely without any scraping sound sound or feel. All the speakers with field coils that I have seen have the hum cancelling winding embedded within the field coil, so most likely the extra "coil" (transformer) is an audio output transformer. As mentioned above put a DC current through the field coil, and the voice coil to the audio output of a radio, etc. Maybe use a variable DC supply to supply current to the field coil. A bulb in series is a good idea so you don't accidentally fry the field coil. If it sounds good at low and high volumes, you've got a good speaker!
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