I think they were NOT installed.
One reference that comes to mind is:
February 1968 Popular HOT rodding
'Drag testing the HOT 375-HP Camaro'
Page 37
Eng photo clearly shows a lack of Cooling decal.
Other may know for sure.
(of course I put one on my 68 during a 90's resto) resto houses were selling so many of us incorrectly applied them.
I need to remove it!
Jim