I believe you're right - paint flaking off of aluminum.
I remember my uncle, a detective with Metro Police, bringing his 427 '69 by the house to show it off in '69. Wish I had taken pictures of it at the time -
A few days afterward, a friend of my sister brought his next car by the house - a '68 SS427 Impala convertible, triple black car (my parents had a '68 327 Impala Custom Coupe at the time). I can still remember that one in my "Cars I Wish I Could Have Had" list.
Those were the days -
Regards,
Steve