Lead In Von Schweikert Speakers

Why is there lead in some of the Von Schweikert speakers

Used for mass-loading. Helps tighten the bass. In the case of VS speakers, it helps a lot.
This is VS' way of bringing down costs. (eg shipping weight). The cavity in the speaker acts as a resonance trap. The lead shot is required in that it tightens the bass, and stabilizes the impedance curve.