It's an African cap. I believe he got the first one from an African tribal leader...but my memory of the story is from a long time ago. I think I read it in his guitar collection book,the big one like Rick Nielsons. That book is hard to find now,without spend big bux. It might be in the smaller book that has his cars in it too. Both books have a lot of great stories told in Billys groovy vernacular.
My guess is that he is bald-ish. He is old as fuck,but still brings it hard...one greasey-groovy note at a time.