I'm no body man, but let me attempt the explanation.
In the days before time

the only body filler available was lead. Responds to moderate heat that will not warp sheet metal. You can grind it and shape it easily using hand tools. Primer adheres to hit just like metal.
So, 'leading' has become a generic term.
With complete idiots now running government, 'lead' has become the latest excuse to confiscate money.
The ONLY hazard lead presents is if it it INGESTED!
Thanks to the EPA and their never ending efforts to ban just about everything, the industry has started selling lead free 'lead.'
All the properties of real lead...it just costs more..
Remember the thread about the Mustang built to honor the Mustang fighter of WWII fame? The reason it was bright silver which was unpainted aluminum is that late in the war we ran out of lead. Lead added to paint was the only way that the paint would adhere to metal.
