I wonder if this engine which probably has a ton of miles on it had some block oil passages?
How regularly was the oil/filter changed over the decades? Dirty oil causes the most damage to engines. Oil with suspended particulates in it would be more accurately call slurry. Slurry is often used as a cutting tool.
If zinc was the main issue then wouldn't the wear would be fairly even on most if not all of the lobes?
For you it isn't relevant since the cam is toast and needs to be replaced. But oil flow,...or a lack of oil flow, or block oil passageways might be
a more accurate cause.
Again it doesn't make much difference to you since the cam is toast no matter what happened.