Here's an extract from FMVSS regulation 108, effective date of January 1, 1968:
S7.8.5 When activated in a steady-burning state, headlamps shall not
have any styling ornament or other feature, such as a translucent cover
or grill, in front of the lens.
But I have to find the complete document....
No doubt the E-types leaked etc, but the 68 regs meant they retooled the bonnet shell.