I'm trying to determine what the speaker type is in an Ampeg B-15N Thiele cabinet - the speaker has a large square ceramic magnet - the entire speaker is black. On the back of the magnet there are three rows of text: 6930009 4327129 8-OHMS Not sure what the 2nd line implies. The 1st line leads me to believe the speaker was manufactured late 1969... I could be wrong. Any ideas what kind of speaker this is?