Sentence:  He [Larry Penberthy] feels metal ice axes are the only safe ones. ... If you do purchase a wood-shafted ax, examine the grain carefully. It should be straight, free of knots and flaws, and continue unbroken from head to <WSD>ferrule</WSD>. Rub it often with linseed oil. 
Meaning of the word marked with <WSD> tags:  The metal spike at the end of the shaft of an ice axe.