[G6Tanuki]

The 'seized' was probably rust between the coil and the concentric magnet pole-pieces - you do suggest that it may have been stored in sub-optimal conditions, and any dampness is likely to cause rust.

Some people have had success by way of 'exercising' the coil for a few hours through feeding it with enough 50Hz AC to cause it to oscillate in and out over 1/2 inch or so in order to work-away the rust. Only problem here is that the rust particles are themselves magnetic and so are going to stay entrained in the areas where the magnetic field's the strongest! There's not really much you can do to get the magnetically-coagulated rust-dust out of the gap.