here is an inductance vs voltage graph at 100 Hz,
usually we use 20 Hz for this test for audio transformers to get an idea about bass response, but since this is for guitar we used a frequency close to the A string on the guitar.
a butt stacked core would have a flatter curve, but since this core was stacked in a semi gapped fashion, we have a different curve.
most people think this is a 500 milli-Henry inductor but it looks a bit higher than that, maybe they spec the inductor at 1 KC.