The hollowed form of the blade on many African spearheads is apparently a by product of using stone/pebble anvils. 
 Ref.M.J.Swanton 1973 
The Spearheads of the Anglo-Saxon Settlements 
ISBN 0 903986 01 9 
 Chap 6 
footnote 6 
 Where this feature is noted and discussed as appearing on both Anglo-Saxon and African spearheads. 
 I don't normaly bother much with referrences but some people seem to like them.
		 
		
		
		
		
		
		
		
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