I think the link's between ferrule and aglet, through their similar use as "a metal ring or cap placed around a pole or shaft for reinforcement or to prevent splitting" (definition for ferrule) and "a tag or sheath, as of plastic, on the end of a lace, cord, or ribbon to facilitate its passing through eyelet holes" (aglet). The only difference seems to be in the material and the exact function, but the idea's roughly the same.