Here are a couple knocker references:
http://geology.about.com/library/bl/images/blknockers.htm

and pictures:
http://community.webshots.com/album/12765328GkwwqaQBPL

Don's picture didn't quite look like a knocker to me though... more of a plug.. but it's so hard to tell in pictures... you'de have to get up close and personal with the rock..

Canie