I have a couple of old Duncan Whistler spin tops. It's been a while since I've thrown one, but from what I remember, it is somewhat hollow, with a couple of holes drilled around the perimeter at the tops widest point. What I don't remember is if the top has a metal disc with a hole in the middle that was recessed into the top of the top. I know that was a big part of getting the wooden Duncan Whistler yo-yo's to whistle.
If I can remember, I'll take a picture tonight and upload it for you if someone else hasn't already answered your question.
Matt