[Phlogiston]

The bell that covers the field coil will come off. Sometimes it is difficult. On many of these speakers the spider is on top of the cone with a screw going to the pole piece (round metal piece inside of voice coil). It does not appear that your speaker has this kid of spider - instead being below the cone. But it does look like the pole piece is pressed into the bell. This is not unusual. Either removing 2-4 nuts and bolts or grinding a couple of rivets should free the assembly revealing the field coil windings.

Also, at ~700 ohms, this would be a fairly small FC with less turns of magnet wire than most others.