It's a capacitor... a passive "crossover", half of the archaeic coils & caps duo, it cuts the low frequencies out.

More about them here - http://www.bcae1.com/passxovr.htm - watch out though, lots of math and science on that page.

I skimmed through the page but didn't read it closely, so it may be full of misinformation - just did a quick serach for coils and caps on Google.

Bren R.